This sounds similar to the question at https://discussions.agilebits.com/discussion/105084/why-do-i-have-to-enter-my-master-password-for-every-login-on-ios-and-how-do-i-fix-it#latest.
Ever since the beginning, I have been able to use the fingerprint authentication on my iPhone to unlock 1P. However, that is stopped. I have to enter the master pw into the app every time. Even if there is some problem with the login and i have to start over, I have to again enter my master pw.
1Password Version: 6.8.9
Extension Version: 18.104.22.168
OS Version: 10.14.5, iOS 12.3.1
Sync Type: dropbox