I purchased a QE50Q80AATXZT (QLED 50" 2021 production) in June '21.
At a glance everything looked ok but after summer I tried to watch my vacation footages. For the first time, reproducing videos through usb keys, I noticed a train of little dark clouds on upper screen limit when displaying a uniform sky or snow, especially annoiyng during camera panning. I named this effect as "dirty screen" without knowing that it's already famous !
Well, the Samsung Assistance was very kind and understanding about my problem. 2 screens were replaced but at the end nothing changed. One of that men explained frankly to me that it is a limit of that back-illumination system. One improvement has been mini-LED of neoQLED but the solution is the future micro-LED (still too expensive).
But now I want to point over one aspect : the DSE is not so noticeable on normal broadcasting signal with a similar sky uniformity on the upper screen limit.
My question : what about signal amplification from the external sources (hdmi & usb) ? Ok, they are very bright but less power could help to avoid Dirty Screen Effect.