I have a similar weird problem on my TCL.
It keeps turning the brightness slider back down.
When I change it to another mode from smart HDR then I don't have this problem
I have a Phillips non-android TV and it is also buggy as all hell
Have you found the solution for this?
This is driving me crazy and of course Philips support doesn't know how to solve.
Sounds like a bug or an OLED tv which doesn't have backlight. Maybe there is a software update available.