diff --git a/src/Analyser/ExprHandler/AssignHandler.php b/src/Analyser/ExprHandler/AssignHandler.php
index 76c40b1ae3..4e02b7bfcb 100644
--- a/src/Analyser/ExprHandler/AssignHandler.php
+++ b/src/Analyser/ExprHandler/AssignHandler.php
@@ -940,7 +940,9 @@ private function produceArrayDimFetchAssignValueToWrite(array $dimFetchStack, ar
 			$offsetValueTypeStack[] = $offsetValueType;
 		}
 
-		foreach (array_reverse($offsetTypes) as $i => [$offsetType]) {
+		$reversedOffsetTypes = array_reverse($offsetTypes);
+		$lastOffsetIndex = count($reversedOffsetTypes) - 1;
+		foreach ($reversedOffsetTypes as $i => [$offsetType]) {
 			/** @var Type $offsetValueType */
 			$offsetValueType = array_pop($offsetValueTypeStack);
 			if (
@@ -981,7 +983,19 @@ private function produceArrayDimFetchAssignValueToWrite(array $dimFetchStack, ar
 				}
 
 			} else {
-				$valueToWrite = $offsetValueType->setOffsetValueType($offsetType, $valueToWrite, $i === 0);
+				// we iterate the offset-types in reversed order.
+				$isLastDimFetchInChain = $i === 0;
+				$isFirstDimFetchInChain = $i === $lastOffsetIndex;
+
+				$unionValues = $isLastDimFetchInChain;
+				if (
+					!$isLastDimFetchInChain
+					&& $isFirstDimFetchInChain
+					&& $offsetType !== null
+				) {
+					$unionValues = $this->shouldUnionExistingItemType($offsetValueType, $valueToWrite);
+				}
+				$valueToWrite = $offsetValueType->setOffsetValueType($offsetType, $valueToWrite, $unionValues);
 			}
 
 			if ($arrayDimFetch === null || !$offsetValueType->isList()->yes()) {
@@ -1076,4 +1090,34 @@ private function isSameVariable(Expr $a, Expr $b): bool
 		return false;
 	}
 
+	private function shouldUnionExistingItemType(Type $offsetValueType, Type $composedValue): bool
+	{
+		$existingItemType = $offsetValueType->getIterableValueType();
+
+		if (!$existingItemType->isConstantArray()->yes() || !$composedValue->isConstantArray()->yes()) {
+			return false;
+		}
+
+		foreach ($existingItemType->getConstantArrays() as $existingArray) {
+			foreach ($existingArray->getKeyTypes() as $i => $keyType) {
+				if ($composedValue->hasOffsetValueType($keyType)->no()) {
+					continue;
+				}
+				$existingValue = $existingArray->getValueTypes()[$i];
+				$newValue = $composedValue->getOffsetValueType($keyType);
+
+				if ($existingValue->isSuperTypeOf($newValue)->yes()) {
+					continue;
+				}
+				if ($newValue->isSuperTypeOf($existingValue)->yes()) {
+					continue;
+				}
+
+				return true;
+			}
+		}
+
+		return false;
+	}
+
 }
diff --git a/tests/PHPStan/Analyser/nsrt/bug-13623.php b/tests/PHPStan/Analyser/nsrt/bug-13623.php
new file mode 100644
index 0000000000..7d473a6830
--- /dev/null
+++ b/tests/PHPStan/Analyser/nsrt/bug-13623.php
@@ -0,0 +1,24 @@
+<?php
+
+declare(strict_types = 1);
+
+namespace Bug13623;
+
+use function PHPStan\Testing\assertType;
+
+function (array $results): void {
+	$customers = [];
+
+	foreach ($results as $row) {
+		$customers[$row['customer_id']] ??= [];
+		$customers[$row['customer_id']]['orders'] ??= [];
+		$customers[$row['customer_id']]['orders'][$row['order_id']] ??= [];
+
+		$customers[$row['customer_id']]['orders'][$row['order_id']]['balance_forward'] ??= 0;
+		$customers[$row['customer_id']]['orders'][$row['order_id']]['new_invoice'] ??= 0;
+		$customers[$row['customer_id']]['orders'][$row['order_id']]['payments'] ??= 0;
+		$customers[$row['customer_id']]['orders'][$row['order_id']]['balance'] ??= $row['order_total'];
+	}
+
+	assertType("array<array{orders: non-empty-array<array{balance_forward: 0, new_invoice: 0, payments: 0, balance: mixed}>}>", $customers);
+};
diff --git a/tests/PHPStan/Analyser/nsrt/bug-13857.php b/tests/PHPStan/Analyser/nsrt/bug-13857.php
new file mode 100644
index 0000000000..737f17763e
--- /dev/null
+++ b/tests/PHPStan/Analyser/nsrt/bug-13857.php
@@ -0,0 +1,67 @@
+<?php
+
+declare(strict_types = 1);
+
+namespace Bug13857;
+
+use function PHPStan\Testing\assertType;
+
+/**
+ * @param array<int, array{state: string}> $array
+ */
+function test(array $array, int $id): void {
+	$array[$id]['state'] = 'foo';
+	// only one element was set to 'foo', not all of them.
+	assertType("non-empty-array<int, array{state: string}>", $array);
+}
+
+/**
+ * @param array<int, array{state?: string}> $array
+ */
+function testMaybe(array $array, int $id): void {
+	$array[$id]['state'] = 'foo';
+	// only one element was set to 'foo', not all of them.
+	assertType("non-empty-array<int, array{state?: string}>", $array);
+}
+
+/**
+ * @param array<int, array{state: string|bool}> $array
+ */
+function testUnionValue(array $array, int $id): void {
+	$array[$id]['state'] = 'foo';
+	// only one element was set to 'foo', not all of them.
+	assertType("non-empty-array<int, array{state: bool|string}>", $array);
+}
+
+/**
+ * @param array<int, array{state: string}|array{foo: int}> $array
+ */
+function testUnionArray(array $array, int $id): void {
+	$array[$id]['state'] = 'foo';
+	// only one element was set to 'foo', not all of them.
+	assertType("non-empty-array<int, non-empty-array{foo?: int, state?: string}>", $array);
+}
+
+/**
+ * @param array<int, array{state: string}|array{foo: int}> $array
+ */
+function testUnionArrayDifferentType(array $array, int $id): void {
+	$array[$id]['state'] = true;
+	assertType("non-empty-array<int, array{state: string}|non-empty-array{foo?: int, state?: true}>", $array);
+}
+
+/**
+ * @param array<int, array{state: 'foo'}> $array
+ */
+function testConstantArray(array $array, int $id): void {
+	$array[$id]['state'] = 'bar';
+	assertType("non-empty-array<int, array{state: 'bar'}|array{state: 'foo'}>", $array);
+}
+
+/**
+ * @param array<int, array{state: 'foo'}> $array
+ */
+function testConstantArrayNonScalarAssign(array $array, int $id, bool $b): void {
+	$array[$id]['state'] = $b;
+	assertType("non-empty-array<int, array{state: 'foo'}|array{state: bool}>", $array);
+}
diff --git a/tests/PHPStan/Analyser/nsrt/bug-8270.php b/tests/PHPStan/Analyser/nsrt/bug-8270.php
new file mode 100644
index 0000000000..f8bf03e4a1
--- /dev/null
+++ b/tests/PHPStan/Analyser/nsrt/bug-8270.php
@@ -0,0 +1,47 @@
+<?php
+
+declare(strict_types = 1);
+
+namespace Bug8270;
+
+use function PHPStan\Testing\assertType;
+
+function doFoo() {
+	/** @var non-empty-list<array{test: false, value: int}> $list */
+	$list = [];
+	$list[0]['test'] = true;
+
+	foreach ($list as $item) {
+		assertType('array{test: bool, value: int}', $item);
+		if ($item['test']) {
+			assertType('true', $item['test']);
+			echo $item['value'];
+		}
+	}
+}
+
+function doBar() {
+	$list = [];
+
+	for ($i = 0; $i < 10; $i++) {
+		$list[] = [
+			'test' => false,
+			'value' => rand(),
+		];
+	}
+
+	if ($list === []) {
+		return;
+	}
+
+	$k = array_key_first($list);
+	assertType('int<0, max>', $k);
+	$list[$k]['test'] = true;
+
+	foreach ($list as $item) {
+		assertType('array{test: bool, value: int<0, max>}', $item);
+		if ($item['test']) {
+			echo $item['value'];
+		}
+	}
+}
diff --git a/tests/PHPStan/Analyser/nsrt/shopware-connection-profiler.php b/tests/PHPStan/Analyser/nsrt/shopware-connection-profiler.php
index ced23ea733..b28e473cc3 100644
--- a/tests/PHPStan/Analyser/nsrt/shopware-connection-profiler.php
+++ b/tests/PHPStan/Analyser/nsrt/shopware-connection-profiler.php
@@ -29,7 +29,7 @@ public function getGroupedQueries(): void
 					$connectionGroupedQueries[$key]['index'] = $i; // "Explain query" relies on query index in 'queries'.
 				}
 
-				assertType("array<string, array{sql: string, executionMS: 0, types: array<int|string, int|Shopware\Core\Profiling\Doctrine\ParameterType>, count: 0, index: int}|array{sql: string, executionMS: float, types: array<int|string, int|Shopware\Core\Profiling\Doctrine\ParameterType>, count: int<1, max>, index: int}>", $connectionGroupedQueries);
+				assertType("array<string, array{sql: string, executionMS: 0|float, types: array<int|string, int|Shopware\Core\Profiling\Doctrine\ParameterType>, count: int<0, max>, index: int}>", $connectionGroupedQueries);
 				$connectionGroupedQueries[$key]['executionMS'] += $query['executionMS'];
 				assertType("non-empty-array<string, array{sql: string, executionMS: float, types: array<int|string, int|Shopware\Core\Profiling\Doctrine\ParameterType>, count: int<0, max>, index: int}>", $connectionGroupedQueries);
 				++$connectionGroupedQueries[$key]['count'];
diff --git a/tests/PHPStan/Rules/Arrays/data/bug-11679.php b/tests/PHPStan/Rules/Arrays/data/bug-11679.php
index 463362516a..1255d3d8e4 100644
--- a/tests/PHPStan/Rules/Arrays/data/bug-11679.php
+++ b/tests/PHPStan/Rules/Arrays/data/bug-11679.php
@@ -32,6 +32,7 @@ public function sayHello(int $index): bool
 		if (!isset($this->arr[$index]['foo'])) {
 			$this->arr[$index]['foo'] = true;
 			assertType('non-empty-array<int, array{foo: true}>', $this->arr);
+			assertType('array{foo: true}', $this->arr[$index]);
 		}
 		assertType('array<int, array{foo?: bool}>', $this->arr);
 		return $this->arr[$index]['foo']; // PHPStan does not realize 'foo' is set
diff --git a/tests/PHPStan/Rules/Comparison/IfConstantConditionRuleTest.php b/tests/PHPStan/Rules/Comparison/IfConstantConditionRuleTest.php
index 79b7b484e1..480b24f472 100644
--- a/tests/PHPStan/Rules/Comparison/IfConstantConditionRuleTest.php
+++ b/tests/PHPStan/Rules/Comparison/IfConstantConditionRuleTest.php
@@ -213,4 +213,10 @@ public function testBug4284(): void
 		]);
 	}
 
+	public function testBug8270(): void
+	{
+		$this->treatPhpDocTypesAsCertain = true;
+		$this->analyse([__DIR__ . '/../../Analyser/nsrt/bug-8270.php'], []);
+	}
+
 }
diff --git a/tests/PHPStan/Rules/Variables/NullCoalesceRuleTest.php b/tests/PHPStan/Rules/Variables/NullCoalesceRuleTest.php
index 1833e4f681..918a6ade23 100644
--- a/tests/PHPStan/Rules/Variables/NullCoalesceRuleTest.php
+++ b/tests/PHPStan/Rules/Variables/NullCoalesceRuleTest.php
@@ -377,4 +377,9 @@ public function testBug13921(): void
 		]);
 	}
 
+	public function testBug13623(): void
+	{
+		$this->analyse([__DIR__ . '/../../Analyser/nsrt/bug-13623.php'], []);
+	}
+
 }