With the iOS app, the Unlock with Face ID option only appears to require Face ID on the initial launch of the application. When switching away from and back to the app, Face ID is not required. Other apps (e.g, 2FAS) require Face ID to unlock every time the app is focused. The latter approach seems like it would provide slightly better protection.
I tested how long the BW Authenticator would remain unlocked after Touch ID unlock on my iPad, and it’s at least 3 hours. (I didn’t test longer.) That is much too long and it’s a security issue.
(There is another, very similar request: Add immediate session timeouts)