Windows 10 Desktop with Firefox 65.0.1, have the same problem.

Dug into firefox' user files. It seems to save the name of the form field the username was extracted from, and may only fill matching form fields later. For my local PixPet entry, the username field name was empty, probably from saving while registering, not on first login.

So a workaround could be deleting the saved credentials and saving them again on next login. Tried it, does not work. Firefox does not detect any username field for credential saving, even on the login page.