ASP.NET Core Attribute Routing bug?

Question

All of my routes in ASP.NET Core 3.1 work fine, except this empty one.

I should be able to hit this route:

• /Document/HB89Letter and hit the Show page
• /Document/HB89Letter/Show works
• /Document/HB89Letter/ does not

This is my code:

[Route("Document/HB89Letter")]
[Authorize]
public class HB89LetterController : MacController
{
    [Route("")]
    [Route("Show")]
    public ActionResult Show(HB89LetterViewModel vm)
    {
        // some code
    }
}