On the official server, I was gathering experience for an enchantment. I finally reached my goal of 30, then when I went to the overworld through my elven portal, all of it was lost. Then, hoping to fix it, I returned to ME and still had 0 experience levels.
I tested it. It is a display bug. You still have the 30 xp levels, but they are not shown. They will be shown when you get xp the next time.
Unfortunately the enchantment table still thinks that you have 0 xp. Before it is fixed, the solution is to kill anything after travelling ot the overworld to get xp before using the enchantment table.