Unable to use PIN or fingerprints at start up

My LG G5 request PIN at start up. It always work except for twice where my PIN won't work and I have to do a hard reset and losing all my data. For the second time, I tried locking into my Google account and using Google's Android Device Manager to reset the PIN with the lock feature (I suspect the PIN somehow mysteriously got reset to something else), but I Google said I already have a screen lock (PIN) and adding lock code is not needed. So I am stuck again other than doing a hard restart. Local repair shop won't take my service because I lost my purchase receipt. Has anyone experience the same issue and have a work around? Thank you.