On my Macbook (Big Sur, but also now on Monterrey) using Safari 15.1 1P does not fill in username/password fields anymore.
It offers me the right entries but I cannot select them.
Also using 1P Mini (auto complete) does not fill in the forms.
It does work with Chrome 95.0.4638.54
Version 7.9.1.BETA-3 (70901003)
1Password Version: 7.9.1 beta3
Extension Version: 7.9.1 beta3
OS Version: MacOS 12.0.1