I’ve noticed some kind of multiplication on my OnePlus 9 too, with v4.2, but for me it only surprised me when rebooting, which I’ve had to do a lot during the initial setup. All I could see was for relatively dark indoor values pre-reboot it would shoot up to about 75% post-reboot.
I didn’t tolerate that for too long, tried to transfer Velis Auto Brightness v5.0.1 over from my old phone, saw it wasn’t working on Android 16 anymore, found Tideo, used that for a bit, got annoyed with its responsiveness issues and weirder and more limited controls after a while, and went back to Velis since it went to v6.xx just this September as the developer restarted work on it and made it viable on newer Androids. Now I’ve got my comfortable reaction curve again, fast response according to my millisecond setting, but also doesn’t suck as much CPU power as Tideo.