A really long time ago I unlocked all three hidden artifact appearances for priest. I still have the ability to transmog them on my current priest, but the original priest I unlocked everything on is long gone. Yet despite the appearances being moggable for me, the achievement doesn’t pop.
Is this a bug? Or am I going to have to go through it all again? Is it even possible to find hidden skins more than once?
I only got the achievements once I logged onto the character that had unlocked the hidden appearance, so it’s possible you’d need to obtain it on another priest. You can definitely get them a second time on another character, I looted one on remix that I had from retail. The easiest one is probably the shadow priest drop from emerald nightmare, but make sure you unlock the artifact first. Now is a good time to farm since the raids are on a daily lockout during remix. You could also make another priest in remix as the drop rates seemed pretty good there.