I'll go ahead and add to this thread instead of starting a new one:
mongodb.com
google.com
Neither of them show password autofill options. Both work on Safari.
Both sites have you enter the username, click next, then type the password before clicking the login button. Could be related?
ETA: Another site I just noticed it on is demiplane.com. The password button shows on the password field but not the username field. If you fill in the password, the username is left blank.