I posted a similar message on steam forums. I tried pasting a link here but this editor seems to have issues with firefox / pasting / adding images.
Basically, your units will try to attack an enemy dungeon heart even if it is invulnerable due to a map objective (ie. game mode is king of the hills and only underhills can destroy the heart). Your units won't stop attacking the heart and need to be recalled to get them off of it.
I had a rally flag in a room near the enemy dungeon heart and half my units went to the flag and the other half started attacking the invuln/shielded enemy dungeon heart.
Solution: if a dungeon heart is shielded/invulnerable, then it shouldn't be targetable by your minions.
Customer support service by UserEcho