Getting out of combat

I wanted to try out a warlock(lvl 4 atm) but ive been experiencing this annoying bug with my pet.

Whenever me and my pet start attacking the same target , the mob often goes out of combat , returns to its original position and goes to 100%(we arent even moving). Seems it happens when the mob switches its target from my pet to me or vice versa. I can't really play like this. I waste twice the time because i have to keep reengaging

server : Loarderon

I had the same issue. I solved it by simply making sure I landed the first hit on the target instead of my imp. So you see a mob you want to kill, start casting shadow bolt, send your pet to attack halfway through your cast so your shadow bolt hits before your imp's fire bolt.
Also the issue seems to stop once you get the voidcaller.