Made an account to report the same issue is happening to me.
On iPadOS 15, pin works like a charm but Face ID biometrics don’t work that well along with Bitwarden… at least without requesting every single time the master password (reporting it to be wrong).
I guess I’ll stick with the pin for now but would like to see this working flawlessly like on Android devices, it’s a joy to use the fingerprint reader there to unlock Bitwarden on autofills.
And for the record, yes, I did try the steps suggested here and at other message boards.