If an user uses Face ID/Touch ID to unlock Bitwarden, then the app should detect a change of Face ID and request the password once to reactivate Face ID. This is a common practice for high security case like password or banking apps.
Otherwise an user with knowledge of the pin code can add his face to Face ID and get access to the Bitwarden vault.
Example of LastPass implementation: https://i.imgur.com/sD0yy3Q.jpg
