On the first stage in the bamboo forest (the one where fog is introduced) you can get softlocked in the fight and have to restart the mission. If one of the cursed trees runs out of its poison attack, it won't move. If it runs out in the fog, you won't be able to kill it since the cursed trees have 0% chance to hit while in the fog.
It is technically possible to kill a stuck tree with alice's doll self-destruct, but if you haven't hit the tree at all its health is too high for you to kill it with dolls.
The easiest fix I can think of would be to have the trees still path towards your units even when they're out of spells.