Have you changed your Universal Login (UL) configuration recently? This method (screen_hint) only works with New UL, and will not work if you have customized the login page, turning on Classic UL.
If you can confirm you are using New UL, can you please also inspect the request and confirm that the screen_hint param is making it as part of the request?
To clarify, the screen_hint parameter is not expected to work with Classic UL. I am trying to confirm that you are using New Universal Login. Can you please post a screen shot of your login page?