I did some testing yesterday with my set. Watching the football, i wanted to see if the screen dimmed as ive read a lot about it also but never seen it with my normal SDR/HDR/dovi settings and neither in games.
I put the TV into imax enhanced mode, brightness max, contrast 90 Gamma -2 peak luminance high, adv contrast high, colour 60 expert 1 and live colour high. It was very bright but for football i dont mind. I did see a dimming in the brighness it wasnt that noticable more so as i was looking for it but changing to and back in picture setting did show how the whites really brightened upped and dimmed. im not sure i would have noticed much if it hadnt have been for the white.
For reference, i normal have all additional settings off, brighness 20 peak luminence low or off for a dim lit room for SDR, change to expert 2 for HDR and the auto settings that also change.