Light sensor tests pass after a fresh boot, reporting values between 23 and 16383. Adaptive brightness works, too, maybe the adjustment range appears a bit narrow to me.
I have run CSD tests now and then since I received the phone, and I don’t seem to remember the Light sensor ever failing. (Other tests have and still do.) My intuition would be that the on-boot behaviour/state is more relevant for this specific problem, than whatever happens during phone use, but I will keep an eye on this during the day.