Came on this morning to make a thread about this and yours was top post, ha.
I just replicated the issue 5 times in a row with the phone sitting flat on my desk at work.
Simply put my finger on the Touch ID - phone instantly unlocks to home screen - 3, 2, 1 and screen changes brightness abruptly. I use auto-brightness and happen to like tru-tone and use it almost all of the time - but, Ive never noticed this happen on my 9.7" iPad Pro.
It is minor; however, can be distracting as myself and others are clearly stopping to think about it.
The thing that bothers me is that - I'd then lock the screen. And I could repeat the above process and it happens time and time again.
What I thought it might be is that if you were in a certain environment - put your phone in your pocket or bag, etc - then moved to another environment - the sensors would take a second to get acclimated and adjust accordingly. THAT I could understand. But, the fact that I can replicate it over and over and over again in the same spot with no changes in lighting - that makes me scratch my head a bit.
hopefully, someone who knows more about this stuff than I do can come up with something as to why it's happening - or it a defect?
mehhhhhhhh lol
I really am enjoying my iPhone 8+ otherwise.