When i was trying to give myself a negative levels. Then the bar just'd disappear until another time you give another negative levels. This happen to same versions of 1.8 snapshots.

Steps:
1. If you have cheating on inside your world, you can proceed. If not, open lan mode with "Cheating on"... Then proceed to step 2
2. Type on chat "/xp 1000000000L <your player>" then it gives a negative level without the bar showing off...

To fix this glitch off, just do "/xp 1000000000L <your player>" again until the Level bar goes back off to 0.

If this glitch was already reported, then please take care of this problem.