Testing F22 Workstation Alpha. On the login screen, I click "Not Listed?" and then click Sign In without entering any username. The Sign In button changes to read Next, and the session selection gear menu appears next to it. Then I click Next, and it changes to read Sign In again. Then I click Sign In, and it takes me back to the login screen.

I was going to say the expected behavior would be to prohibit pressing the button if no characters have been entered in the username entry, but:

halfline:  well blank username technically is "legal"
halfline:  but maybe we should forbid it anyway
halfline:  and clearly the state machine is getting kind of hosed up from your description
halfline:  but yea, one thing about pam is the input username doesn't have to equal the output username, and some pam modules treat blank username special (though none of the ones we care about for the log in as another user feature)