Custom controller selector and attribute routing

I develop WebAPI application and I need to use a custom controller selector, so I have created this one:

public class CustomControllerSelector : DefaultHttpControllerSelector
{
    (...)
    
    public override HttpControllerDescriptor SelectController(HttpRequestMessage request)
    {
        // It's just a simplified logic which exemplify the problem
        var controller =_controllerAssembly.GetType("App.Controllers.MyController");
        return new HttpControllerDescriptor(this.httpConfiguration, controller.Name, controller);
    }
}

My controller contains two GET methods:

public class MyController : System.Web.Http.ApiController
{
    [Route("~/api/myController/GetAll")]
    public IEnumerable<MyModel> GetAll(){ ... }

    public MyModel Get(){ ... }
}

Unfortunately, when I go to http://localhost/api/myController/, I get the error:

Multiple actions were found that match the request:

GetAll on type App.Controllers.MyController

Get on type App.Controllers.MyController"

But if I go to http://localhost/api/myController/GetAll, it works as intended and collection of object is returned.

It is worth mentioning that if I don't use the custom controller selector, it works like a charm and both actions gives valid response.

Do you know why custom controller selector (derived from default selector) causes this problem?

Workaround:

Since the original (default) controller selector is initialized properly and works as expected, I aggregate it instead of inherit from it.

class CustomControllerSelector : IHttpControllerSelector 
{ 
    private DefaultHttpControllerSelector defaultSelector; 

    public CustomControllerSelector(DefaultHttpControllerSelector defaultSelector)
    { 
        (...) 
}