This issue has been brought up various times by users on AVSForums (2021 Samsung QN90A Neo Qled 4K Series Owners Thread (No Price Talk) | Page 592 | AVS Forum).
The issue is that local dimming standard and high do not work correctly in game mode when xbox series x or ps5 are connected. Icons/menus/hud/text are washed out and colors are dull. This is only fixed when setting local dimming to low. It is temporarily fixed when you press the menu button on the remote, which causes the whole screen to brighten up which then fixes the colors. Firmware 1901 has not fixed this.
SAMSUNG please look into this issue, game mode local dimming should work flawlessly on a $1400 flagship TV and the issue has been prevalent throughout various firmware levels.
Agree, it's horrible. These luminance fluctuation in game mode are even mentioned by Vincent from hdtv test in his QN95A/QN90A review at the 10:40 minutes mark.
Please Samsung do something about it. These tv cannot be recommended to anybody taking image quality seriously