I have a brand new Note 20 Ultra with the flicker issue right from the start. I've searched everywhere for solutions but none have worked so far.
It looks like the quick flicker you get when taking a screenshot, except it starts from the top left corner rather than all around the screen and is not quite as bright.
Appears using any app and also on the home screen
The only other thing I have noticed is that if I press the 'recents' icon the Messaging app shows - even if I haven't used Messages (including from a fresh forced reboot) but if I tap Messaging it disappears from recents but the app doesn't open.
The last resort is the full factory reset but I see no point as the issue has occurred from the first time I used the phone.
[Edit] Other people also have the same problem (https://us.community.samsung.com/t5/Note20/Note-20-latest-update-and-issues/m-p/2006513#M31467) so it would appear it's not just my new phone.
They describe the issue better than I can. I saw a video online of the "flicker" issues and my problem is not that - a "flash" is a better description.
Solved! Go to Solution.
Wipe cache partition.
Boot to safe mode and test
If OK: factory reset.
If not: flash with Odin.
If not: disassembly, disconnect battery, hold power button, connect and test.
If not: defective screen.
Proceed at your own risk
Good point and that was on the list. But as it appeared a Samsung update issue, it was likely that another new phone would have the same problem
The issue was solved as above - Messages app "always on top" creating some sort of conflict. Disabled "always on top" and now issue appears fixed.