Having had this same problem multiple times, I actually made it a point to save a “login” for those sites that when I autofill it reminds me which auth service I’ve used.
Username: Log in with FB
Password: <blank>
Username: Log in with FB
Password: <blank>
There are good use cases where third party logins are good enough.
I've tried pinging the developers to support the idea directly, but I've been met with incomprehension.
Not sure if I'm explaining it wrong, or if it's way more work than I'm anticipating.