Try to use https://support.1password.com/change-website-password/
Generated value always replace current. password! Tried before focus on current, new, confirm fields- no result. Pls check my video.
Tried at Twitter and GitHub
PS Also tested with Chrome 86 + ext220.127.116.11
1Password Version: 1Password 7 Version 7.6 (70600005)
Extension Version: Safari
OS Version: OS X 10.15.7
Sync Type: 1pass+iCloud