Touch ID suddenly broke on 2 devices (iPhone 6 and 7)

Hey there,

I'm reaching out to the larger community to better understand what's happening here. Both phones are "hand-me-downs" that my family bought new and passed on down from the parents to the kids after a while. All I know is that one morning my youngest phone 6 owner tells me that touch id was no longer working. The phone displayed an error message after every restart that it failed to activate touch id. I tried for a few hours without success to re-activate touch id.

A few weeks later the same story with an iPhone 7.

I read many funky articles suggesting to clean the phone and other funky stuff (not sure how dirty some people let their phones get that it breaks touch id for that reason???) but nobody had any suggestions for my case. I tried a full reset and reinstall but to no avail.

Apple isn't very outspoken about the problem either. In the Apple support note below, I fall into the "Still not working? Call Apple support!" section.

https://support.apple.com/en-us/HT207537

What I really want to know (before I invest several hours to make an appointment and to get in line at a crowded Apple Store only to be told that my device is out of warranty - which they all are as they are more than a year old and I don't have Apple Care plans for any of them) is if anyone has gone through this process and could elaborate on what they found out?

Are touch id buttons failing after a while? Is this a known hardware issue? Has anyone gone through the Apple Store process and gotten the issue fixed through software/configuration by an Apple Genius?

Thank y'all for any light you can shed on this.

Cheers.

Volker