Can confirm that this bug also exists in the Warrior Dungeon.
It seems to be related to the Allied Magus Soldier, but indirectly - possibly.
I've never encountered it in the Gunner Dungeon, only Assassin and Warrior, so it might somehow be related to the Revenge ability. Some kind of memory overflow, perhaps?
Anyway, there might be a possible workaround: not to use Reflect Shield as the first ability in your turn, in other words: perform an attack with another character, then use the shield. Needs more testing though.
EDIT: Also, never seems to occur when using autoplay.