Something suspiciously like this happened to me on a second-hand iPhone 6 I bought a few weeks ago. Talk about crappy timing.
Everything pointed to a software issue, but every repair person I took it to (both apple and non-apple) kept saying it was a hardware fault.
Touch ID stopped working and the phone drains super quickly despite not being in use. Hoping Apple can provide a "fix", not gonna hold my breath though.
I got a second-hand 5S last year with faulty Touch ID, the seller admitted that it didn't work and didn't even notice because he never used it (at the time also many Touch IDs stopped working after iOS 8 updates). I got a lower price and a friend took it to Apple Store in Germany (had couple months warranty left), where they tried to change the display first, then swapped it for a new unit without issues.
Everything pointed to a software issue, but every repair person I took it to (both apple and non-apple) kept saying it was a hardware fault.
Touch ID stopped working and the phone drains super quickly despite not being in use. Hoping Apple can provide a "fix", not gonna hold my breath though.