Late 2015 21 inch 4K. Mid 2018 had a playback problem with iTunes movies, luckily had AppleCare so contacted Apple support and then started the problems. I live in Thailand and at the time had no Apple Store, only approved resellers or repairers. Apple asked me to take it into a local approved reseller who sent it to Bangkok. It was returned two weeks later with a new display, no charge as under warranty. Drove home, plugged it in exactly as sent, phoned Apple, told to take it back and very sorry for inconvenience. Another two weeks and went to collect it, this time plugged it in at the resellers, paperwork said new motherboard fitted, switch on and still the same problem. I had sent a letter, in Thai language telling them which movies were problems and they just ignored it. Another phone call, the Lady who was looking after the case said she was requesting a replacement but please allow them to check it for the third time , sent to Bangkok again but nothing done, then the email saying nothing else they can do but wait for next update. The update solved the problem and so I thought good, new display, new motherboard and now I have the pink edge screen !!!!!
I will be following this thread as I have very little hope of getting anything done here, maybe a trip to Bangkok will be necessary, we'll see.
Note, first Apple was a Mac 1986, lasted for ages, now have Mac Air, MacBook Pro, iPad Air, iPod, iPhone, I must be mad !.