title: ASP.NET Core 3.0 Endpoint Routing doesn't work for default route
body: <p>I have migrated an existing API project from <code>2.2</code> to <code>3.0</code> based on guidelines from <a href="https://docs.microsoft.com/en-us/aspnet/core/migration/22-to-30?view=aspnetcore-3.0&amp;tabs=visual-studio" rel="noreferrer">this page</a>.</p>
  
  <p>Thus I've removed:</p>
  
  <pre><code>app.UseMvc(options =&gt;
  {
      options.MapRoute("Default", "{controller=Default}/{action=Index}/{id?}");
  });
  </code></pre>
  
  <p>and inserted:</p>
  
  <pre><code>app.UseEndpoints(endpoints =&gt;
  {
      endpoints.MapControllerRoute(name: "Default", pattern: "{controller=Default}/{action=Index}/{id?}");
  });
  </code></pre>
  
  <p>But no controller and action would be bound. All I get for any API I call is 404.</p>
  
  <p>How should I debug it and what do I miss here?</p>
  
  <p><strong>Update:</strong> The <code>Startup.cs</code> file is located in another assembly. We reuse a centralized <code>Startup.cs</code> file across many projects.</p>