I recently purchased the Halo Mash-Up Pack, and soon after, I decided to get on Mineplex. It appeared to me that my skins were not appearing. I rejoined, and my skins still weren't appearing. From there, I put on the default Steve Skin as well as some other skins, like from Skin Pack 1, and these skins appeared, but not the Halo ones. 

It is my belief (although I could be wrong) that skins like Halo have blocks added onto them, such as how Emile has the big shoulder pad with a knife and Kat has a robotic arm, it's possible these skins may conflict with something on Mineplex, and as a result, removes my skin altogether.

(I should also note that, yes, the pictures were taken on iOS, my Xbox One was inaccessible at the time, however I assure you this happens on Xbox)