Able to fix it finally. This is for future reference, if it helps anyone :
I removed the screen protector and tried setting up the fingerprint. It had the same 50% reset problem with one finger, but I tried multiple fingers to set up the first fingerprint and then set up a second one using just my desired finger and deleted the first fingerprint. It worked. But then putting the screen protector back on, I couldn't unlock the phone with fingerprint. But I did another fingerprint set up with the TG on, and this time it worked.
A little weird process, but eventually I am able to use fingerprint scanner.