WebApi Proxy throwing 404 for URLs with invalid chars

I'm using C# .NET WebApi to create an API proxy, here is basic code that I have that works fine for most calls:

[HttpGet]
[Route("api/proxy/{*.}")]
public async Task<HttpResponseMessage> Get()
{
   // some route here...
}

But when I have a URL with special chars, like : (%3F) or / (%2F) it fails with 404. If I remove the special chars, it works.

http://localhost:3000/api/proxy/viewing/v1/items/urn%3Aadsk.viewing%3Afs.file%3AdXJuOmVUE_dmVyc2lvbj0x%2Foutput%2Fd4b6ef1c-8d219bd84c9d%2F0.pf?domain=http%3A%2F%2Flocalhost%3A3000

And tried a few suggestions on web.config, but nothing:

<system.web>
  <httpRuntime targetFramework="4.6" requestPathInvalidCharacters=""/>
  <pages validateRequest="false" />
</system.web>

All other WebApi endpoints are working with a Global.asax like this:

public class Global : System.Web.HttpApplication
{
  protected void Application_Start(object sender, EventArgs e)
  {
    GlobalConfiguration.Configure(Config.WebApiConfig.Register);
  }
}

And the Config.WebApiConfig:

public class WebApiConfig
{
  public static void Register(HttpConfiguration config)
  {
    config.MapHttpAttributeRoutes();
  }
}