That’s because there’s a polyfill in place that makes bitwarden happy, but that polyfill is only in place on the main login screen (e.g. it won’t work for adding a new user or changing password).
That polyfill does nothing to satisfy Safari/iOS, which requires the autocomplete attribute.
FWIW I’m still having issues with 1Password on Safari Mobile, though not on the desktop Safari. Both are the latest iOS and macOS versions.
1Password definitely has the right username and password, since login on desktop works fine. But it pastes in on Safari mobile and HA rejects it as incorrect. If I type the exact same letters manually it works.
I don’t have this issue with any other mobile web apps I can think of. Only HA on iOS Safari.