So we know certain enemies will bug if you kill them in one hit. For example the wights in the Fornost Wraith of Fire- if you one hit kill them, the door behind them doesn't open and the instance hangs. You have to do a small hit, then a big one so combat triggers and then the door opens on death.

Because of that, for certain boss enemies I often send in my LM pet to do some small damage before I use a big attack. However I've been noticing that my pet's damage isn't registering. For example:

(start of fight, boss at full health)
Pet attacks: floating damage says -350
(boss is still at full health)
Wait for the next pet attack.
Pet attacks: floating damage says -274
(boss is still at full health)
Wait for the next pet attack.
Pet attacks: floating damage says -421
(boss is still at full health)
A few seconds later the boss's health finally drops by a small amount.

What's with that?