There seems to be a bug with PvP, where if you are attacked by a parrallel phalanx, your guys will simply hold their ground, not fight back, and die. If you tell them to attack, they will stopt this. Though, as you said, this will make them break formation. You can also try turning of defensive formation, but this too will result in a broken formation. This is not a EB bug but a RTW bug...