So, as a workaround until they fix this, will moving up next to a monster and then choosing to attack it avoid this behavior? (Just be sure auto-end-turn is off.)
For what it's worth, I never allow attack orders to run past the end of a turn (I like to control my units turn-by-turn for everything except long marches through safe territory) and, in something like 120 hours of play, I've never encountered this problem in the context of an attack.
I have once encountered a similar problem, when I ordered a multi-turn march to deliver the dragon eyes and the quest location didn't activate when my stack arrived. Since then, I've always moved stacks up adjacent to quest locations (as well as empty lairs, lootables, or any other type of "react on entry" space) and have not had any similar problems when doing it that way.
So, yes, it sounds like that probably would be an effective workaround, based on my experience.