TestServer HttpClient SendAsync returns NOT_FOUND when api is in classLibrary

[ghstahl]

Describe the bug

The exact same controller works when it is in the main application OR in a class library.
i.e.
https://localhost:5001/api/SomeThing2/dog

However, when called from TestServer's HttpClient it returns a 404 when the SomeThing2 controller is in a class library. Cut and paste the controller back to the main application results in TestServer's HttpClient returning a 200.

To Reproduce

minimal project
or
AzurePipelinesDemoAspNetCore

Currently, it is failing my tests when the Something2 is an external project
https://github.com/P7CoreOrg/AzurePipelinesDemoAspNetCore/blob/master/src/SomeThing2/Controllers/SomeThing2Controller.cs

Cut and Paste the SomeThing2Controller to the following location;
https://github.com/P7CoreOrg/AzurePipelinesDemoAspNetCore/tree/master/src/TheWebApp/Controllers
which is in the main app make everything work.

https://github.com/P7CoreOrg/AzurePipelinesDemoAspNetCore/blob/master/src/xUnit_TheWebAppTests/UnitTestSomeThing.cs

       [Fact]
       public async Task Test_Get_SomeThing2_Success()
       {
           var client = _fixture.Client;
           var req = new HttpRequestMessage(HttpMethod.Get, "/api/SomeThing2/dog")
           {
               // Content = new FormUrlEncodedContent(dict)
           };
           var response = await client.SendAsync(req);
           response.StatusCode.ShouldBe(System.Net.HttpStatusCode.OK);
           var jsonString = await response.Content.ReadAsStringAsync();
           jsonString.ShouldNotBeNullOrWhiteSpace();

           var dogs = JsonConvert.DeserializeObject<List<string>>(jsonString);
           dogs.ShouldNotBeNull();
           dogs.Count.ShouldBeGreaterThan(0);

       }

Expected behavior

Should not return a 404 when SomeThing2Controller is in a separate classlibrary.

[ghstahl]

The only clue I have so far from stepping symbols is EndpointDataSource contains only endpoints from the host app and not the referenced projects.

[mkArtakMSFT]

Thanks for contacting us, @ghstahl.
This got fixed today: #4332
Closing this issue as a dupe.