I have never been able to unlock my under-screen fingerprint reader by taking it out of my pocket. This is because the reader isn't in a good position when the phone is in my pocket. Yes, it's where my thumb is when properly holding my phone, but my grip is different when pulling the phone out of my pocket. My older phone with the reader on the back had my index finger in position before I even attempted to take out of my pocket. It was slower at reading my prints, but was always unlocked before I looked at the screen.

I'd love to use the old phone for so many reasons, but the lack of updates has rendered it useless. No Lineage or Graphene for that one either.