Received my S21U yesterday and noticed a weird thing happens when vivid mode is on.
Basically, the colours change when apps take up the whole screen. It's mostly a slight change but it's really visible on skin tones. On my device when things are full screen, people's faces look pale, yellow and washed out. This disappears immediately if I put the app into pop-up view, splitscreen, or if I open another pop up app over the top. It also disappears if I swipe up to go to recent apps and the current app minimizes. It's annoying because the skin tones look normal and healthy when not full screen. Then as soon I full screen whatever I'm looking at the colours change straight away before my eyes and I see the washed out yellow skin tones appear. This doesn't happen on natural mode or when eye comfort is on, it only happens with vivid.
The same issue happened on my Note 20 Ultra, but the yellowed/washed out colours were SO much worse that I ended up returning it. Oddly enough it doesn't happen on my Tab S7+.
If you have a spare minute can you see if this happens on your device? Open this random image of the legend William Hung full screen with vivid mode on. Then try swiping up to see recent apps and then tapping the app to full screen the image again. If the same thing happens on your device you should notice the skin tones change. He should look normal when the window isn't full but on full screen he looks really yellow and ill. The same should happen if you open it in split screen or popup view or open another app in pop up view over the top.
Anyone else noticed this or know why it happens? It actually kind of annoys me as the skin tones look ugly and at this price point I expect everything to be perfect.
I FINALLY FOUND THE SOLUTION!
After months of trying to solve this issue on my wife's A7, I managed to find the fix. I knew it had to be a software problem. A sort of filter, based on the selective nature of the color cast on different apps / system screens.
Just go to Accessibility > Visibility Enhancements > Colour Adjustment > On
Then play with the options until you get the skin tones color you like.
Hope this works for you as it was driving me crazy.