diff --git a/src/State/Provider/ContentNegotiationProvider.php b/src/State/Provider/ContentNegotiationProvider.php
index d9f242ce85e..2f842fdf831 100644
--- a/src/State/Provider/ContentNegotiationProvider.php
+++ b/src/State/Provider/ContentNegotiationProvider.php
@@ -67,7 +67,9 @@ public function provide(Operation $operation, array $uriVariables = [], array $c
     private function addRequestFormats(Request $request, array $formats): void
     {
         foreach ($formats as $format => $mimeTypes) {
-            $request->setFormat($format, (array) $mimeTypes);
+            $existingMimeTypes = $request->getMimeTypes($format);
+            $newMimeTypes = array_unique(array_merge((array) $mimeTypes, $existingMimeTypes));
+            $request->setFormat($format, $newMimeTypes);
         }
     }
 
diff --git a/tests/State/Provider/ContentNegotiationProviderTest.php b/tests/State/Provider/ContentNegotiationProviderTest.php
index dd610bd4b8d..9094bac7949 100644
--- a/tests/State/Provider/ContentNegotiationProviderTest.php
+++ b/tests/State/Provider/ContentNegotiationProviderTest.php
@@ -27,6 +27,23 @@ class ContentNegotiationProviderTest extends TestCase
 {
     use ProphecyTrait;
 
+    public function testAddRequestFormatsAddsMimeTypesToFormat(): void
+    {
+        $provider = new ContentNegotiationProvider();
+        $request = new Request();
+        $formats = ['json' => ['application/problem+json']];
+
+        $operation = new Post(outputFormats: $formats);
+        $context = ['request' => $request];
+
+        $provider->provide($operation, [], $context);
+
+        $mimeTypes = $request->getMimeTypes('json');
+
+        $this->assertContains('application/problem+json', $mimeTypes);
+        $this->assertContains('application/json', $mimeTypes);
+    }
+
     public function testRequestWithEmptyContentType(): void
     {
         $expectedResult = new \stdClass();