diff --git a/src/Analyser/ExprHandler/AssignHandler.php b/src/Analyser/ExprHandler/AssignHandler.php
index 10b1f135eb..1baaa08905 100644
--- a/src/Analyser/ExprHandler/AssignHandler.php
+++ b/src/Analyser/ExprHandler/AssignHandler.php
@@ -938,7 +938,7 @@ private function isImplicitArrayCreation(array $dimFetchStack, Scope $scope): Tr
 
 	/**
 	 * @param list<ArrayDimFetch> $dimFetchStack
-	 * @param list<array{Type|null, ArrayDimFetch}> $offsetTypes
+	 * @param non-empty-list<array{Type|null, ArrayDimFetch}> $offsetTypes
 	 *
 	 * @return array{Type, list<array{Expr, Type}>}
 	 */
@@ -947,10 +947,11 @@ private function produceArrayDimFetchAssignValueToWrite(array $dimFetchStack, ar
 		$originalValueToWrite = $valueToWrite;
 
 		$offsetValueTypeStack = [$offsetValueType];
+		$generalizeOnWrite = $offsetTypes[array_key_last($offsetTypes)][0] !== null;
 		foreach (array_slice($offsetTypes, 0, -1) as [$offsetType, $dimFetch]) {
 			if ($offsetType === null) {
 				$offsetValueType = new ConstantArrayType([], []);
-
+				$generalizeOnWrite = false;
 			} else {
 				$has = $offsetValueType->hasOffsetValueType($offsetType);
 				if ($has->yes()) {
@@ -959,9 +960,11 @@ private function produceArrayDimFetchAssignValueToWrite(array $dimFetchStack, ar
 					if (!$scope->hasExpressionType($dimFetch)->yes()) {
 						$offsetValueType = TypeCombinator::union($offsetValueType->getOffsetValueType($offsetType), new ConstantArrayType([], []));
 					} else {
+						$generalizeOnWrite = false;
 						$offsetValueType = $offsetValueType->getOffsetValueType($offsetType);
 					}
 				} else {
+					$generalizeOnWrite = false;
 					$offsetValueType = new ConstantArrayType([], []);
 				}
 			}
@@ -1010,7 +1013,24 @@ private function produceArrayDimFetchAssignValueToWrite(array $dimFetchStack, ar
 				}
 
 			} else {
-				$valueToWrite = $offsetValueType->setOffsetValueType($offsetType, $valueToWrite, $i === 0);
+				// when $unionValues=false the array item-type will be replaced with $valueToWrite
+				// when $unionValues=true the existing array item-type will be union'ed with $valueToWrite -> type gets wider
+				$unionValues = false;
+				if ($i === 0) {
+					$unionValues = true;
+				} elseif (
+					$generalizeOnWrite
+					&& $i === count($offsetTypes) - 1
+					&&
+						(
+							$originalValueToWrite->isConstantScalarValue()->yes()
+							|| !$offsetValueType->getIterableValueType()->isSuperTypeOf($valueToWrite)->yes()
+						)
+				) {
+					$unionValues = true;
+				}
+
+				$valueToWrite = $offsetValueType->setOffsetValueType($offsetType, $valueToWrite, $unionValues);
 			}
 
 			if ($arrayDimFetch === null || !$offsetValueType->isList()->yes()) {
diff --git a/tests/PHPStan/Analyser/nsrt/assign-nested-arrays.php b/tests/PHPStan/Analyser/nsrt/assign-nested-arrays.php
index 31cecaa4d1..3de574894a 100644
--- a/tests/PHPStan/Analyser/nsrt/assign-nested-arrays.php
+++ b/tests/PHPStan/Analyser/nsrt/assign-nested-arrays.php
@@ -12,8 +12,9 @@ public function doFoo(int $i)
 		$array = [];
 
 		$array[$i]['bar'] = 1;
-		$array[$i]['baz'] = 2;
+		assertType('non-empty-array<int, array{bar: 1}>', $array);
 
+		$array[$i]['baz'] = 2;
 		assertType('non-empty-array<int, array{bar: 1, baz: 2}>', $array);
 	}
 
diff --git a/tests/PHPStan/Analyser/nsrt/bug-10089.php b/tests/PHPStan/Analyser/nsrt/bug-10089.php
new file mode 100644
index 0000000000..21122cdfc3
--- /dev/null
+++ b/tests/PHPStan/Analyser/nsrt/bug-10089.php
@@ -0,0 +1,44 @@
+<?php
+
+namespace Bug10089;
+
+
+use function PHPStan\Testing\assertType;
+
+class Test
+{
+
+	protected function create_matrix(int $size): array
+	{
+		$size = min(8, $size);
+		$matrix = [];
+		for ($i = 0; $i < $size; $i++) {
+			$matrix[] = array_fill(0, $size, 0);
+		}
+
+		// array<int<0, max>, non-empty-array<int, 0>>
+		assertType('list<non-empty-list<0>>', $matrix);
+
+		$matrix[$size - 1][8] = 3;
+
+		// non-empty-array<int, non-empty-array<int, 0|3>&hasOffsetValue(8, 3)>
+		assertType('non-empty-list<non-empty-array<int<0, max>, 0|3>>', $matrix);
+
+		for ($i = 0; $i <= $size; $i++) {
+			if ($matrix[$i][8] === 0) {
+				// ...
+			}
+			if ($matrix[8][$i] === 0) {
+				// ...
+			}
+			if ($matrix[$size - 1 - $i][8] === 0) {
+				// ...
+			}
+		}
+
+		return $matrix;
+	}
+
+}
+
+
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/Rules/Arrays/data/bug-11679.php b/tests/PHPStan/Rules/Arrays/data/bug-11679.php
index 463362516a..46374b8cb2 100644
--- a/tests/PHPStan/Rules/Arrays/data/bug-11679.php
+++ b/tests/PHPStan/Rules/Arrays/data/bug-11679.php
@@ -31,7 +31,8 @@ public function sayHello(int $index): bool
 		assertType('array<int, array{foo?: bool}>', $this->arr);
 		if (!isset($this->arr[$index]['foo'])) {
 			$this->arr[$index]['foo'] = true;
-			assertType('non-empty-array<int, array{foo: true}>', $this->arr);
+			assertType('non-empty-array<int, array{foo?: bool}>', $this->arr);
+			assertType('true', $this->arr[$index]['foo']);
 		}
 		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/StrictComparisonOfDifferentTypesRuleTest.php b/tests/PHPStan/Rules/Comparison/StrictComparisonOfDifferentTypesRuleTest.php
index 2450cb19b3..d5e7f59fd8 100644
--- a/tests/PHPStan/Rules/Comparison/StrictComparisonOfDifferentTypesRuleTest.php
+++ b/tests/PHPStan/Rules/Comparison/StrictComparisonOfDifferentTypesRuleTest.php
@@ -1046,6 +1046,11 @@ public function testBug13282(): void
 		$this->analyse([__DIR__ . '/../../Analyser/nsrt/bug-13282.php'], []);
 	}
 
+	public function testBug10089(): void
+	{
+		$this->analyse([__DIR__ . '/../../Analyser/nsrt/bug-10089.php'], []);
+	}
+
 	public function testBug11609(): void
 	{
 		$this->analyse([__DIR__ . '/data/bug-11609.php'], [