At the start of an encounter if a character is forced to move, such as when they start outside of the borders of the battle, and that character passed through another; the character that was passed through will be unable to move. I've also seen a character that should have moved not move at all and be stuck outside of the battle for it's duration.
This happens to me most often in the reverse Y formation, as this is the longest formation and the borders are much too close to the encounter start. This forced movement also makes the use of formations pointless because it breaks that formation.
I'd suggest removing the borders completely. Forcing encounters to happen in close quarters arbitrability severely limits tactical choices. This makes the player feel that they have to fight an encounter in a way that was chosen for them rather than how they've chosen to. This has the benefit of also adding to the fun of replaying the game, picking different ways to engage each encounter. At the very least moving the borders back somewhat from where the encounter starts would fix this bug and make how to engage the encounters feel less forced.