From reading the user manual and white paper. The master password is deleted from the apple keychain if Face ID fails to identify you few times. I think there should be a warning that you have x amount of tries left before Face ID will be disabled. The whitepaper says 5 failed attempts, the manual says 3...
https://www.enpass.io/docs/security-whitepaper-enpass/quick_unlock.html
"Also, if five consecutive attempts to quick unlock are unsuccessful, Enpass erases the master password from the iOS keychain and prompts to enter master password to unlock Enpass. On next successful attempt, it is saved again in the iOS keychain. When you disable Quick Unlock from Enpass settings, Enpass erases the master password from the iOS keychain."