Welcome to being a Samsung Q90T owner. I have went through 3 panels and all have this. Not to mention the horirble game mode and 120hz/vrr funcationality. Last samsung I ever purchase, and to think i paid $2800 for this TV....
That's an unusual amount of DSE. Does it bother you when you are viewing your normal content at your normal viewing distance? If so return or exchange that thing while you still can. Keep in mind, though, that you'll never get a perfect panel, and your next could be worse. It's a "panel lottery."
I have two 55Q90Ts that are much cleaner vis-a-vis DSE, although they have dead pixels, which infuriates me. I've gone through 9 Samsung panels in 2020. Honestly, I would pay Samsung $500 more than the cost of the panel if they could gaurantee minimal DSE and no dead pixels; it's such a horrendous waste of time.
I came across your post because I have the exact same grey uniformity issue as you do. The pictures you uploaded are almost identical to mine. For the most part it's not noticeable, until I watched Dunkirk last night, when during panning shots with a white background it was clearly visible and distracting. This also occurs during all sport broadcasts. It feels like I'm having to compromise with this TV, which is frustrating after having spent such money on it.
Have you been able to resolve this issue at all? Surely they can't keep replacing your panels with ones with the same defect.
No, I think it is across the board (just part of how it is built). Even the Rtings review seems to have it in their grey uniformity test. Like you said, it's only noticeable on very bright uniform screens, so I've just moved on from the issue. Seems to be just different trade-offs with other big screen TVs (e.g. I've heard LGs can have dark grey uniformity issues).