Recently, after creating a brand new world on xbox and playing with a few friends of mine they have told me that items and mobs that they could see previously vanish from the world and are completly invisible to them, this however does not effect the host (me) and I can see everything fine.

Leaving and rejoining to redownload the add-ons (since now they have to do it everytime they join) will temporarily fix the issue but items and mobs will disappear overtime for other players, as far as I can tell this only effects Xbox and happens with any add-on, this makes it incredibly hard to enjoy any custom content. 

If you want to reproduce this bug download any custom add-on pack, preferably one that includes new items and mobs and have another player join the world, wait a minute or two and said items and mobs should vanish.

(Updated reply in comments below)