I have never seen a bug like that mentioned. The experience stops when enemies are 3 levels or lower than you. As for being quite challenging, if you are playing on nightmare, even mooks are going to give you a challenge. If you are not playing on nightmare or hard, then I would suggest you are probably using under-leveled gear if it is taking you more than a minute to wipe out a group of 3-4 enemies. Either that or you need to learn how the combat mechanics function to make sure your warriors are doing enough armor penetration to actually hurt the enemies. Abilities only magnify damage after armor reduction.
Here is a thread on combat mechanics:
http://forum.bioware.com/topic/543549-guide-documentation-for-combat-mechanics/
You can understand what the attributes are really doing (Strength is essentially useless on a warrior, Willpower is better), and better understand what abilities are most effective, and especially how damage is calculated so you can try to maximize your character build.
If you would like some tips on combat, this forum has lots of helpful posters:
http://forum.bioware.com/forum/1188-combat-strategy/