To prevent this you need to redirect the root to /RDWeb.
On the RD Web Access server(s) open Internet Information Services (IIS) Manager (it’s under Administrative Tools).
Expand the tree and click Default Web Site, then open the “HTTP Redirect” app:
Fill in the redirect path, don’t forget to check “Only redirect requests to content in this directory”, and click apply.
That’s it. No need to reset IIS. This is tested to be working on Windows 2012 and Windows 2012 R2 versions of RD Web Access.
As an added bonus, HTTP requests are redirected to HTTPS as well.