Xp orb limit issue

In playing minecraft yesterday i was working with a mob water farm on a zombie spawner. I let the spawner run for 1 hour and 15 minutes. In killing all the zombies I went from level 5 to 27.

I then let the farm run for 8 hours while the game was very laggy I managed to kill all the zombies in the farm area and went from level 27 to only 42.

I think there is a bug in how many xp orbs can be created or how many the player can absorb at one moment in time I am unsure which.

I can provide additional details and screenshots if needed.