another bug i encountered was: i created a healer sub and i picked "shilen elder" so i started at level 40. i went to cruma tower, 3rd floor. at level 40-41 lets say i killed 15 monsters to reach 42. at level 41-42 (this is i think where the bug is) i killed maybe 40-50 mobs to reach level 42. at level 42-43 lets say i only killed 20-25 mobs to reach level 43. the number of mobs i killed above is only approximate to emphasize that you need to kill more mobs at level 42 than level 43.
a matter of approaching the same lvl with the mob on the spot? the closer the lvl gap betwen player and mob the better the xp he is receiveing.