Try clicking ‘Login’ for the website (your email) with the boxes blank and it should fail but may then populate the boxes for you.
Some website logins do behave like that, they won’t auto-populate the login boxes at first, but will do it following the first fail.
It isn’t the browser or your saved passwords, it’s that website.
There again some websites, (particularly banking sites etc), will not let you save or auto-fill logins/passwords at all for security reasons.