Mine is also with this problem, I am thinking of suing Sasmung judicially, because in my country we only have 7 days to return it and this period has already expired. Besides, this problem was the samsung itself that caused the last firmware.
Thought I was going to get a Q90T for PS5 and Xbox Series X, Guess im not, if the game mode is bad and having issues with local dimming.
There was update 1402.1 today
Ok I only had time to try out one game and I tried warzone since I'm very familiar with it. It was a good test because I can see the black screens and local dimming performance.
Ok so when it comes to brightness. I had game motion plus on previously. I turned it off because I expect the tv to work without it. So just loading in to the game I immediately noticed that I actually had a black in the loading screen with the big warzone lighting and soldier. Before the update, it was a completely blue screen. I used to have to turn on game motion plus to get a LESS BLUE black but I can confirm that now with game motion plus off, performance of local dimming is far better and I see legitimate black levels. I then went into the gunsmith where it previews your gun with a nearly black background. There too I saw legitimate black levels that far exceed what we had before.
For the brightness I loaded into the campaign mission "highway of death" which is very bright. It was brighter than it was before. However I want to make this clear. I had contrast enhancer on high previously update so that did not change. I don't know how the tv was before it so if it got dinner with it off after the update I wouldn't know. What made it brighter for me was turning of game motion plus. So I would say brightness is most likely unchanged. As for the picture I easily noticed it's more dynamic. Highlights are brighter and darker areas remain dark with way less blooming than before. It's way better and more of what I expected of this tv.
I also want to tell you all that I did not have the dimming bug in game mode. Mine only did it in the standard picture preset but I don't have time to test that now.
Lastly I want to say I'm doing this mid day in a moderately lit room. I can notice that local dimming performance is much better because blacks would look blue even during this time of day pre update. I will do more testing at night when I have time.