Never tested it but why would you care? Is it really too bright for you? I think everyone just lets it do its thing. They are way to worried about their CPU throttling...
It actually does bother me. It is a bit bright. Minimum brightness is just a few shades less bright but it makes all the difference! My previous 7 Plus didn’t do this, and currently in my family the 8 Plus, 6S, and iPad Pro 10.5 all when put to minimum brightness in a pitch dark room, retain that minimum brightness despite being unlocked several times. If I turn a light on, they all adjust automatically fine too.
If I set the brightness to minimum (an empty brightness bar) in a pitch dark room, lock my phone, and unlock it again with the same environment, it shouldn’t automatically adjust itself to be a few shades brighter with the bar just ever so slightly filled.
I just want to know if other iPhone X models are experiencing this issue and if it is common or not (which will help me determine if it is worth going in for an exchange or to live with the screen being just a tad brighter than I would like when in a pitch dark room during bed time).