Fixed the issue. Will be adding the tests as a separate commit.

Author: mkArtakMSFT

src/Mvc/Mvc.ApiExplorer/src/ApiResponseTypeProvider.cs

@@ -363,9 +363,10 @@ internal static void CalculateResponseFormatForType(ApiResponseType apiResponse,
         }
 
         // If the method is declared to return IActionResult, IResult or a derived class, that information
-        // isn't valuable to the formatter.
-        if (typeof(IActionResult).IsAssignableFrom(unwrappedType) ||
-            typeof(IResult).IsAssignableFrom(unwrappedType))
+        // isn't valuable to the formatter, unless it implements IEndpointMetadataProvider interface.
+        bool isIResultOrIActionResult = typeof(IActionResult).IsAssignableFrom(unwrappedType) ||
+            typeof(IResult).IsAssignableFrom(unwrappedType);
+        if (isIResultOrIActionResult && !typeof(IEndpointMetadataProvider).IsAssignableFrom(unwrappedType))
         {
             return null;
         }

src/Mvc/Mvc.Core/src/ApplicationModels/DefaultApplicationModelProvider.cs

@@ -3,7 +3,9 @@
 
 using System.Linq;
 using System.Reflection;
+using Microsoft.AspNetCore.Builder;
 using Microsoft.AspNetCore.Http;
+using Microsoft.AspNetCore.Http.Metadata;
 using Microsoft.AspNetCore.Mvc.ActionConstraints;
 using Microsoft.AspNetCore.Mvc.ApiExplorer;
 using Microsoft.AspNetCore.Mvc.Filters;
@@ -19,6 +21,8 @@ namespace Microsoft.AspNetCore.Mvc.ApplicationModels;
 internal class DefaultApplicationModelProvider : IApplicationModelProvider
 #pragma warning restore CA1852 // Seal internal types
 {
+    private static readonly MethodInfo PopulateMetadataForEndpointMethod = typeof(EndpointMetadataPopulator).GetMethod(nameof(PopulateMetadataForEndpoint), BindingFlags.NonPublic | BindingFlags.Static)!;
+
     private readonly MvcOptions _mvcOptions;
     private readonly IModelMetadataProvider _modelMetadataProvider;
     private readonly Func<ActionContext, bool> _supportsAllRequests;
@@ -349,9 +353,33 @@ internal PropertyModel CreatePropertyModel(PropertyInfo propertyInfo)
         applicableAttributes.AddRange(routeAttributes);
         AddRange(actionModel.Selectors, CreateSelectors(applicableAttributes));
 
+        // There may be additional metadata for the action that is associated with the return type.
+        AddEndpointMetadata(actionModel, methodInfo);
+
         return actionModel;
     }
 
+    private static void AddEndpointMetadata(ActionModel actionModel, MethodInfo methodInfo)
+    {
+        // Get metadata from return type
+        var returnType = methodInfo.ReturnType;
+        if (CoercedAwaitableInfo.IsTypeAwaitable(returnType, out var coercedAwaitableInfo))
+        {
+            returnType = coercedAwaitableInfo.AwaitableInfo.ResultType;
+        }
+
+        if (returnType is not null && typeof(IEndpointMetadataProvider).IsAssignableFrom(returnType))
+        {
+            // Return type implements IEndpointMetadataProvider
+            var builder = new InertEndpointBuilder();
+            var invokeArgs = new object[2];
+            invokeArgs[0] = methodInfo;
+            invokeArgs[1] = builder;
+            PopulateMetadataForEndpointMethod.MakeGenericMethod(returnType).Invoke(null, invokeArgs);
+            actionModel.Properties.Add(typeof(ProducesResponseTypeAttribute), builder.Metadata.ToArray());
+        }
+    }
+
     private string CanonicalizeActionName(string actionName)
     {
         const string Suffix = "Async";
@@ -675,4 +703,18 @@ private static void AddRange<T>(IList<T> list, IEnumerable<T> items)
             list.Add(item);
         }
     }
+
+    private static void PopulateMetadataForEndpoint<T>(MethodInfo method, EndpointBuilder builder)
+        where T : IEndpointMetadataProvider
+    {
+        T.PopulateMetadata(method, builder);
+    }
+
+    private sealed class InertEndpointBuilder : EndpointBuilder
+    {
+        public override Endpoint Build()
+        {
+            return new Endpoint(RequestDelegate, new EndpointMetadataCollection(Metadata), DisplayName);
+        }
+    }
 }