XP bar vanishes after enchanting/using anvil using up all levels.
1) Splash yourself with xp until you have more than one level.
2) switch to survival, and enchant an item to use up all your levels.
3) The xp bar should disappear.
It should have displayed the remaining xp in the bar that was not enough to fill a level.
TESTED IN BOTH SP AND MP, SAME PROBLEM.