When you start attacking a mob or boss that has been in combat before it will continue it’s attack rotation from where it was when it was deagroed.
So if you attack a stafrusher(the stick with a stick that heals itself) and die or leach it right before it starts healing, the first thing it’ll do when re-agroed is start healing.
Same goes for some dungeon bosses, if you wipe right before mist keeper does it’s AoE, it’ll do AoE as soon as you attack it again.
For the most part it’s not that big a deal since a lot of attacks/abilities are on a timer instead of the hitcycle, but for mist keeper it can be really frustrating and annoying, sometimes even cause a wipe as soon as you start the fight(by getting AoE as soon as you jump in and not having enough time/stamina to get back out again).