I've used this skin before while being able to join realms (2nd Birthday Skin Pack) (Conkers Bad Fur Day Conker). And it's now the only skin pack that does this, examples. (Founders Cape Skin Pack, Skin Pack 2, Skin Pack 3, (All I know of so far)). Even When I choose the skin I only used (Conker), I can only play with the skin if "Multiplayer Game" is turned off in my personal survival world.