TLDR: Only the last added fingerprint works.
The problem I had was that the sensor couldn't read my right thumb.
To troubleshoot, I have added, deleted and readded my both thumbprints many many times.
I always added my right thumb first then left thumb during my multiple attempts to troubleshoot.
Finally by chance, I added my left thumb first then right. Now my right thumb works but the left one doesn't. haha.
So i think this might be a software issue because only the most recently added print works to unlock phone.
Try it if you have an issue with one thumb. Good luck.
Edit: I just deleted my right print and my left works again.