I have tried to re-upload my skins, and i've specifically chosen the "slim" model but when i join a server/realm all skins including my own have the black lines on the back and are all the classic steve models. I have looked through other bug reports and i have seen that others have had the same issue. This has been the case across other versions (i have tried 1.8 and 1.12), not just 1.15.2. I have also tried opening minecraft without optifine/forge and the same problem occurs.