diff --git a/src/Analyser/NodeScopeResolver.php b/src/Analyser/NodeScopeResolver.php
index 0135b2dab7..668c3aa9bd 100644
--- a/src/Analyser/NodeScopeResolver.php
+++ b/src/Analyser/NodeScopeResolver.php
@@ -5450,14 +5450,15 @@ private function processAssignVar(
 			$offsetValueType = $varType;
 			$offsetNativeValueType = $varNativeType;
 
-			$valueToWrite = $this->produceArrayDimFetchAssignValueToWrite($offsetTypes, $offsetValueType, $valueToWrite);
+			$valueToWrite = $this->produceArrayDimFetchAssignValueToWrite($dimFetchStack, $offsetTypes, $offsetValueType, $valueToWrite, $scope);
 
 			if (!$offsetValueType->equals($offsetNativeValueType) || !$valueToWrite->equals($nativeValueToWrite)) {
-				$nativeValueToWrite = $this->produceArrayDimFetchAssignValueToWrite($offsetNativeTypes, $offsetNativeValueType, $nativeValueToWrite);
+				$nativeValueToWrite = $this->produceArrayDimFetchAssignValueToWrite($dimFetchStack, $offsetNativeTypes, $offsetNativeValueType, $nativeValueToWrite, $scope);
 			} else {
 				$rewritten = false;
 				foreach ($offsetTypes as $i => $offsetType) {
 					$offsetNativeType = $offsetNativeTypes[$i];
+
 					if ($offsetType === null) {
 						if ($offsetNativeType !== null) {
 							throw new ShouldNotHappenException();
@@ -5471,7 +5472,7 @@ private function processAssignVar(
 						continue;
 					}
 
-					$nativeValueToWrite = $this->produceArrayDimFetchAssignValueToWrite($offsetNativeTypes, $offsetNativeValueType, $nativeValueToWrite);
+					$nativeValueToWrite = $this->produceArrayDimFetchAssignValueToWrite($dimFetchStack, $offsetNativeTypes, $offsetNativeValueType, $nativeValueToWrite, $scope);
 					$rewritten = true;
 					break;
 				}
@@ -5784,9 +5785,10 @@ static function (): void {
 	}
 
 	/**
+	 * @param list<ArrayDimFetch> $dimFetchStack
 	 * @param list<Type|null> $offsetTypes
 	 */
-	private function produceArrayDimFetchAssignValueToWrite(array $offsetTypes, Type $offsetValueType, Type $valueToWrite): Type
+	private function produceArrayDimFetchAssignValueToWrite(array $dimFetchStack, array $offsetTypes, Type $offsetValueType, Type $valueToWrite, Scope $scope): Type
 	{
 		$offsetValueTypeStack = [$offsetValueType];
 		foreach (array_slice($offsetTypes, 0, -1) as $offsetType) {
@@ -5821,6 +5823,31 @@ private function produceArrayDimFetchAssignValueToWrite(array $offsetTypes, Type
 				$offsetValueType = TypeCombinator::intersect($offsetValueType, TypeCombinator::union(...$types));
 			}
 			$valueToWrite = $offsetValueType->setOffsetValueType($offsetType, $valueToWrite, $i === 0);
+
+			$arrayDimFetch = $dimFetchStack[$i] ?? null;
+			if ($arrayDimFetch === null || !$offsetValueType->isList()->yes()) {
+				continue;
+			}
+
+			if ($scope->hasExpressionType($arrayDimFetch)->yes()) { // keep list for $list[$index] assignments
+				$valueToWrite = TypeCombinator::intersect($valueToWrite, new AccessoryArrayListType());
+			} elseif ($arrayDimFetch->dim instanceof BinaryOp\Plus) {
+				if ( // keep list for $list[$index + 1] assignments
+					$arrayDimFetch->dim->right instanceof Variable
+					&& $arrayDimFetch->dim->left instanceof Node\Scalar\Int_
+					&& $arrayDimFetch->dim->left->value === 1
+					&& $scope->hasExpressionType(new ArrayDimFetch($arrayDimFetch->var, $arrayDimFetch->dim->right))->yes()
+				) {
+					$valueToWrite = TypeCombinator::intersect($valueToWrite, new AccessoryArrayListType());
+				} elseif ( // keep list for $list[1 + $index] assignments
+					$arrayDimFetch->dim->left instanceof Variable
+					&& $arrayDimFetch->dim->right instanceof Node\Scalar\Int_
+					&& $arrayDimFetch->dim->right->value === 1
+					&& $scope->hasExpressionType(new ArrayDimFetch($arrayDimFetch->var, $arrayDimFetch->dim->left))->yes()
+				) {
+					$valueToWrite = TypeCombinator::intersect($valueToWrite, new AccessoryArrayListType());
+				}
+			}
 		}
 
 		return $valueToWrite;
diff --git a/tests/PHPStan/Analyser/nsrt/bug-12274.php b/tests/PHPStan/Analyser/nsrt/bug-12274.php
new file mode 100644
index 0000000000..437dc09ae3
--- /dev/null
+++ b/tests/PHPStan/Analyser/nsrt/bug-12274.php
@@ -0,0 +1,109 @@
+<?php declare(strict_types=1);
+
+namespace Bug12274;
+
+use function PHPStan\Testing\assertType;
+
+/**
+ * @param non-empty-list<int> $items
+ *
+ * @return non-empty-list<int>
+ */
+function getItems(array $items): array
+{
+	foreach ($items as $index => $item) {
+		$items[$index] = 1;
+	}
+
+	assertType('non-empty-list<int>', $items);
+	return $items;
+}
+
+/**
+ * @param non-empty-list<int> $items
+ *
+ * @return non-empty-list<int>
+ */
+function getItemsByModifiedIndex(array $items): array
+{
+	foreach ($items as $index => $item) {
+		$index++;
+
+		$items[$index] = 1;
+	}
+
+	assertType('non-empty-array<int<0, max>, int>', $items);
+	return $items;
+}
+
+/** @param list<int> $list */
+function testKeepListAfterIssetIndex(array $list, int $i): void
+{
+	if (isset($list[$i])) {
+		assertType('list<int>', $list);
+		$list[$i] = 21;
+		assertType('non-empty-list<int>', $list);
+		$list[$i+1] = 21;
+		assertType('non-empty-list<int>', $list);
+	}
+	assertType('list<int>', $list);
+}
+
+/** @param list<list<int>> $nestedList */
+function testKeepNestedListAfterIssetIndex(array $nestedList, int $i, int $j): void
+{
+	if (isset($nestedList[$i][$j])) {
+		assertType('list<list<int>>', $nestedList);
+		assertType('list<int>', $nestedList[$i]);
+		$nestedList[$i][$j] = 21;
+		assertType('non-empty-list<non-empty-list<int>>', $nestedList);
+		assertType('non-empty-list<int>', $nestedList[$i]);
+	}
+	assertType('list<list<int>>', $nestedList);
+}
+
+/** @param list<int> $list */
+function testKeepListAfterIssetIndexPlusOne(array $list, int $i): void
+{
+	if (isset($list[$i])) {
+		assertType('list<int>', $list);
+		$list[$i+1] = 21;
+		assertType('non-empty-list<int>', $list);
+	}
+	assertType('list<int>', $list);
+}
+
+/** @param list<int> $list */
+function testKeepListAfterIssetIndexOnePlus(array $list, int $i): void
+{
+	if (isset($list[$i])) {
+		assertType('list<int>', $list);
+		$list[1+$i] = 21;
+		assertType('non-empty-list<int>', $list);
+	}
+	assertType('list<int>', $list);
+}
+
+/** @param list<int> $list */
+function testShouldLooseListbyAst(array $list, int $i): void
+{
+	if (isset($list[$i])) {
+		$i++;
+
+		assertType('list<int>', $list);
+		$list[1+$i] = 21;
+		assertType('non-empty-array<int, int>', $list);
+	}
+	assertType('array<int, int>', $list);
+}
+
+/** @param list<int> $list */
+function testShouldLooseListbyAst2(array $list, int $i): void
+{
+	if (isset($list[$i])) {
+		assertType('list<int>', $list);
+		$list[2+$i] = 21;
+		assertType('non-empty-array<int, int>', $list);
+	}
+	assertType('array<int, int>', $list);
+}
diff --git a/tests/PHPStan/Rules/Functions/ReturnTypeRuleTest.php b/tests/PHPStan/Rules/Functions/ReturnTypeRuleTest.php
index 54206ff2de..12307bf5bc 100644
--- a/tests/PHPStan/Rules/Functions/ReturnTypeRuleTest.php
+++ b/tests/PHPStan/Rules/Functions/ReturnTypeRuleTest.php
@@ -345,4 +345,18 @@ public function testBug11301(): void
 		]);
 	}
 
+	public function testBug12274(): void
+	{
+		$this->checkExplicitMixed = true;
+		$this->checkNullables = true;
+
+		$this->analyse([__DIR__ . '/../../Analyser/nsrt/bug-12274.php'], [
+			[
+				'Function Bug12274\getItemsByModifiedIndex() should return non-empty-list<int> but returns non-empty-array<int<0, max>, int>.',
+				36,
+				'non-empty-array<int<0, max>, int> might not be a list.',
+			],
+		]);
+	}
+
 }