When using dimensional cakes/waypoints to travel between dimensions, any XP vanishes, but it will all reappear upon gaining any XP.
Present in dev 35df953 and live version of the pack.
How to reproduce: Travel between dimensions using cakes or Journeymap waypoints while having XP.

This is an extremely old vanilla bug that's completely out of our control, your XP actually doesn't go anywhere, it just doesn't get displayed properly.
Relog or interact with something that decreases/increases your experience, such as tome of knowledge or XP obelisk.