This was originally posted by AndrewKurtz in the general forum. After playing a bit with the current RC, I have seen it in my games often as well.
It seems to be an unforeseen interaction between the rule which has a stack assume an offensive posture if it is in a region where its side has less than 5% MC, and the new retreat logic where a stack chooses not to retreat outside a region if there is no enemy stack present in an offensive posture.
As an illustrative example, McDowell attacks Mananas in Aug 61 with Beauregard in a defensive posture. McDowell retreats from battle, but does not choose to leave the province because there is no enemy force with an attack stance (as reflected by the battle log). In the next turn resolution, even if the player sets McDowell to passive and orders him out of the region to a completely friendly region, the stack's player issued orders are overridden, McDowell's stack assumes an "offensive" posture, and fights another battle in Mananas. He looses, no enemy in an offensive posture, chooses not to retreat out of region... etc.
It's causing failed attackers to get stuck in 0% friendly MC regions. It might be useful to have a rule that stacks never choose to retreat locally if their side has less than 5% friendly MC.