My S20FE (updated to Android 12) also have screen flickering with 1 vertical green line on screen not frequently. Usually this occur when I double tap the screen to unlock my phone (always display is on). After noticing this issue, I turn off the always display to always display when charging battery only, so far the screen flickering not happening anymore. Hopefully it is just an common issue after updated to android 12 but not worn and tear for my phone just 1year 4months old.