I'm running into consistently bizarre behavior in several battles during a current war. My armies' morale gradually drops during fighting as usual, but then all of a sudden it drops like a stone from around half to near zero. One battle in particular follows this script every time: my army besieging an enemy castle is attacked by their army. Both start at just above half morale. They fight for a day or two with a very small loss of morale, then both instantly drop to a sliver. Following that, I get the "victory" screen saying that my army won the battle, with just a few dozen casualties on each side. Then, instantly, my army disappears and the enemy army is left standing. What is going on?