Just stumbled upon this thread.. My 2018 S developed the yellow ring near the beginning of the year, they cancelled one appointment to address it (COVID staff shortages) then got around to the UV treatment a month before the warranty ran out. Didn't work (though improved it some), tried it again, didn't change anything, now they're back to "your warranty has expired and we wouldn't replace the screen anyway because it's cosmetic".
Guess we'll see if arbitration gets anywhere.