Some Lenovo laptops also have an "adaptive display brightness" setting hidden in the BIOS. Which in the case of my X1 Yoga, at least, is crap. If you leave it enabled it makes your screen brightness waver all over the place allegedly in response to whatever's being displayed, but A) it lags behind screen updates for what feels like over a second, and B) even if you're just displaying a static image it still waffles your screen brightness back and forth sometimes for no identifiable reason.

I have no clue whatsoever who thought this was a good idea.

Sane individuals will probably want to just turn this off. Note, however, that this has nothing to do with any kind of inactivity timeout as OP has described. That's a separate thing. The adaptive brightness with happily turn your screen into a low-grade discotheque even while you're actively using the machine.