diff --git a/src/Psalm/Internal/Analyzer/Statements/Expression/Fetch/ArrayFetchAnalyzer.php b/src/Psalm/Internal/Analyzer/Statements/Expression/Fetch/ArrayFetchAnalyzer.php
index f41844aa9..bb28daf96 100644
--- a/src/Psalm/Internal/Analyzer/Statements/Expression/Fetch/ArrayFetchAnalyzer.php
+++ b/src/Psalm/Internal/Analyzer/Statements/Expression/Fetch/ArrayFetchAnalyzer.php
@@ -444,12 +444,14 @@ class ArrayFetchAnalyzer
 
                 if ($in_assignment
                     && $type instanceof TArray
-                    && $type->type_params[0]->isEmpty()
+                    && (($type->type_params[0]->isEmpty() && $type->type_params[1]->isEmpty())
+                        || ($type->type_params[1]->isMixed() && \is_string($key_value)))
                     && $key_value !== null
                 ) {
+                    $from_mixed_array = $type->type_params[1]->isMixed();
                     // ok, type becomes an ObjectLike
                     $array_type->removeType($type_string);
-                    $type = new ObjectLike([$key_value => new Type\Union([new TEmpty])]);
+                    $type = new ObjectLike([$key_value => $from_mixed_array ? Type::getMixed() : Type::getEmpty()]);
                     $array_type->addType($type);
                 }
 
@@ -616,7 +618,7 @@ class ArrayFetchAnalyzer
 
                             $array_access_type = Type::getMixed();
                         } else {
-                            if (!$context->inside_isset || $type->sealed) {
+                            if ($type->sealed) {
                                 $object_like_keys = array_keys($type->properties);
 
                                 if (count($object_like_keys) === 1) {
diff --git a/tests/ArrayAssignmentTest.php b/tests/ArrayAssignmentTest.php
index 4fa9323d2..7e5e145e0 100644
--- a/tests/ArrayAssignmentTest.php
+++ b/tests/ArrayAssignmentTest.php
@@ -1072,6 +1072,20 @@ class ArrayAssignmentTest extends TestCase
                     $c = new C();
                     $c[] = "hello";',
             ],
+            'addToMixedArray' => [
+                '<?php
+                    /**
+                     * @param array{key: string} $a
+                     */
+                    function foo(array $a): void {
+                        echo $a["key"];
+                    }
+
+                    function bar(array $arr) : void {
+                        $arr["key"] = "qqq";
+                        foo($arr);
+                    }'
+            ],
         ];
     }
 
@@ -1207,9 +1221,11 @@ class ArrayAssignmentTest extends TestCase
                     ];',
                 'error_message' => 'DuplicateArrayKey',
             ],
-            'mixedArrayAssignment' => [
+            'mixedArrayAssignmentOnVariable' => [
                 '<?php
-                    $_GET["foo"][0] = "5";',
+                    function foo(array $arr) : void {
+                        $arr["foo"][0] = "5";
+                    }',
                 'error_message' => 'MixedArrayAssignment',
             ],
             'implementsArrayAccessPreventNullOffset' => [