Most likely, (Samsung doesn't give their technical advise) it's under powered Exynos chipset compatibility issue with latest software update. There is a member here, who got an LCD screen replacement for FREE last year, but eventually the new screen got damaged again. Meaning 2X naka-greenlines yong phone nya.
Better trade-in (N20U valued around 16k) your phone and get a new one. Kung may budget, get an S23-Ultra, so far it's got high technical praises worldwide (watch YT vlogs).
Today, June 5th, i received a software update, thinking okay maybe if i install it the line on my screen will go away, unfortunately, it didn't.
Now i don't know what's really going on🥺
If you try to take a screen shot whilst the faint line is there, it wouldn't show in the picture. It's as if you are taking a screen shot with a normal screen and comes out as a normal photo.
We are sorry to hear about your experience with your Samsung Galaxy Note 20 Ultra having issues with the green line on screen. I feel and understand your situation and if it happens on my device, I will be disappointed too.
Similar to our dear Samsung members' replies, I highly recommend bringing your device to the closest Samsung Authorized Service Center in your area. Rest assured that your device will undergo an assessment to pinpoint the root cause of the issue. They may guide you and offer solutions to restore it.