Whenever I play HDR content (tested in YouTube and files directly) the screen brightness is pretty low, even though I have my brightness set to max. The second I stop playing it switches back to the normal brightness.
Did you try it with and without automatic brightness? Did you check the app’s own brightness setting? On many apps it can be changed by swiping up & down on the left side of the video. Do you use the original YouTube app or something like RVX/Revanced?
With automatic brightness it still happens.
I tried both the original and RVX, no idea how to change the brightness in the app, when I scroll up it only opens the next video carousel.