Google's OAuth2 login_hint parameter does not work with the sub identifier despite what the docs say?

Keywords: oauth oauth-2.0 google-oauth google-oauth2

Question: 

I'm setting login_hint to a previously retrieved sub identifier (unique Google ID) as part of an authorization request url, but it doesn't select the appropriate account. It does do it if instead of the sub I supply the email address of that user.

According to Google's documentation however the sub identifier should also work:

Optional. If your application knows which user is trying to authenticate, it can use this parameter to provide a hint to the Google Authentication Server. The server uses the hint to simplify the login flow either by prefilling the email field in the sign-in form or by selecting the appropriate multi-login session.

Set the parameter value to an email address or sub identifier, which is equivalent to the user's Google ID.

Any ideas how to get it working, or whether this is indeed a bug?

Answers: