I have the Paypal app on my phone and tried filling in the password with 1Password. I see the 1Password controls now placed above the keyboard, which looks nice. I hit the first button there to have it fill in the password for Paypal. 1Password does that, and then the password disappears. If I change the Paypal app to show the password I can see the password come and go when I hit the 1Password button for it. I finally had to bring up the 1Password app, copy the Paypal password, and then paste it into Paypal.
The Paypal app is version 8.2.2.
1Password Version: 7.7.8.BETA-3
Extension Version: Not Provided
OS Version: Android 11