They weren't "working fine", they were on the verge being found out to have been tampered with : aka "not fine".
Also, this message occured in 8.3 so your argument is again bogus.
No tampering. As I've said many times, Apple doesn't care whether it has been tampered with. You can put in a 100% untampered touchid sensor in there and it will throw this error. You can literally take a brand new touchid sensor from a brand new iphone straight from the shrink-wrap, put it into another iphone, and this error will disable the phone.
I forgot which exact version of iOS introduced this error. I know it was somewhat recent. Sorry for giving Apple the benefit of the doubt.