So, I think this only happens when 1P hasn't previously had a passphrase entered and enters a "dormant" mode (in my case: unlock with a PIN).
Open an app or website, use autofill, enter passphrase, select item (maybe automatically selected as a suggestion), then return to app.
Sometimes credentials don't actually fill, so you do autofill again, and enter passphrase, select item, then return to app. Hopefully on second time the creds get filled. Good.
Now, open up 1P or use autofill on another app, and you still have to use passphrase to unlock. Very annoying.
1Password Version: 7.3.2
Extension Version: Not Provided
OS Version: iOS 12.3.1
Sync Type: Not Provided