Are the other units in the stack mounted and ranged? I think what is happening is some amount of kiting going on during the auto-resolve simulation. I don't think there are any map boundaries during the sim and mobile ranged units just tend to repeatedly move back I think. I tend to get a second or two delay whenever I fight one of those archer spider hybrid monsters, that's what I assume is happening though I don't recall actually checking the combat log to confirm. You can probably see what's going on in the log, I'm guessing there are a lot of lines of "Enemy moved toward X" "X moved away from Y" or whatever, and I'd guess this starts once your sov gets a mount and/or a movement buff.
My other theory is roots messing up the sim (those archer spider things usually come with spiders with roots), but this one is less likely since I don't think any active skills are used during the sim, and I don't remember if the assassin gets a root.