Sieges - Why troops stuck and stop fighting?

[thenextking]

Is it a Bug? It happens specially if there is flames on the battlefield. Everytime you try to take a fief many of your soldiers just stuck and do nothing until they get killed. They don't even try to defend theirselves. Sometimes, dozens, if not hundreds, just die without fighting. It happens since previous versions, but when I tried to take deepwoodmotte all my vale house guards were dying without doing nothing to avoid it.