Check the combat log. The imps have resistance to (slashing?) damage, meaning that damage is halved against them.
The cleric things sounds like a bug. Check that it is not healing in multiple instances: i.e., "Character heals for 1d8. Character then heals for 3. Character then heals for an additional 3" instead of "Character heals for [1d8+3+3]"
Disengage: if you try to jump to a place not currently reachable from your position (obstacle/enemy in the way, out of your jump range) your character will move to another position before jumping. This is probably what is triggering the enemy AoO, not the jumping itself. Check the glowing white outline showing where your character will move before you confirm jumping.
Walk Around: If the red threat arrow doesn't pop up but you still get AoO'd, yes that's a bug. But sometimes the arrow is hard to see, and the pathfinding in this game is currently dumb. If this persists, try moving in small steps around the enemy instead of just clicking behind them.
