Apologies if my understanding of how the auth / registration process works, I haven't actually been through this workflow myself because my account was created a few years ago.
I manage a Phabricator installation for our company with probably 300-400 user accounts (just a guess, I don't have a list of users in front of me). Roughly once per week I get a request for help from a new user who is unable to login to their account.
We have Phabricator configured with Google authentication. Registration is disabled, so the process for new starters is that an admin creates an account for them, which senda them a welcome email containing a link to login. I think that what happens is that new users don't realise that they need to explicitly link their Google account to Phabricator (perhaps they assume it was done automatically). At some point in time, their session expires and they are forced to login, which they cannot be as they never linked their account.
As I remember before, my understanding of the NUX process might be wrong, but our users do seem to hit this issue regularly.