I think you're right about the brightness increasing when using the Intelligent Scan / Face Recognition for the unlock. When I turn everything off except the fingerpreint scanner, the brightness doesn't increase. When I turn on intelligent scan, it start's doing it again.
In the facial recognition settings under lock screen and security there is an option that brightens the screen when you unlock you phone. Just turn it off and you wont have to turn on your phone to a really bright screen.
I had the same problem with my S8 and figured it was just my phone being bugged... after upgrading to the S10 I realized it was a setting in the face unlock biometrics which can be disabled if you'd rather enter a pin or fingerprint to unlock your phone when it is dark out. I never really use auto brightness so it was odd and annoying my phone did that to me