On my dev sever webmanifest.php returns a status of 401. Says:
Unauthorized
This server could not verify that you are authorized to access the document requested. Either you supplied the wrong credentials (e.g., bad password), or your browser doesn't understand how to supply the credentials required.
That looks like a web server error, so it's happening before PHP executes and thus before XF is involved. I don't think our code can actually send that response code via the manifest.
