I also have S21 fe but my device doesn't get green line issue as i don't update my phone its heated, i wait till it gets cooldown or i put it in refrigerator for 4-5 minutes then update. Heating leads to pixel burn in display and pixel burn leads to green and pink lines on display.
Kindly complaint to Samsung customer support through Samsung Members App as error report also send log files Along with this error report for this issue. Then go to service center for replace/repair it's. If you like please accept as a solution it's help to others users also,click on 3dots in my reply section and click on accept solution. Thanks.
