Note for others: this issue was resolved 
Basically, for some reason, the Epic mesh rotation is different to the rotation used by a lot of marketplace assets. Because, as part of the transition between first person and third person we have to set this rotation. By default we use the Epic mesh so it sets to that rotation. It’s a very trivial matter to set it up to use new rotations, however I will make it so that even this isn’t necessary in the next update, just to ensure no one else runs into this