This has been an issue for a while now, where the skin randomly stops working. I personally believe that it has to do with a certain Forge update. I couldn't say which one it is, but it happened around December 2022. I'll keep you all posted if I find anything else out.
Edit: Seems to be 1.18+ from my testing.
Edit 2: All I can recommend is installing the "Everlasting Skins" mod, and set the skin to your skin in game.