In several cases, iOS 11 disables the Face ID, just like it did with Touch ID and in a support document published by Apple, today we can read what these cases are
In addition to the standard ones, it also shows a low battery level. It is clear that using Face ID requires a battery and if we are already in the limit, the iPhone will tend to keep it as much as possible, forcing us, however, to manually unlock the code.
Face ID is disabled when:
- The device has just been turned on or restarted.
- The device is not unlocked for more than 48 hours.
- You did not use the code to unlock the device for the last six and a half days and did not use Face ID for the last 4 hours.
- The device received a remote lock command.
- You have tried five times, failing to use Face ID to unlock the device.
- You have started Emergency or SOS Emergency mode by simultaneously pressing one of the two volume keys and the side key for 2 seconds.
- If the battery charge of the device is less than 10%.