Are they equipped with ranged weapons and horses? They go play skirmish whenever so equipped, regardless of any orders to stand their ground, and have done so ever since I've been playing (not just v0.711). If it's not meant to be a feature, it's almost certainly a known issue. Try searching the forums for it.
EDIT: I should add that they've always followed my orders to mount/dismount. I find this useful to a) make them stand their ground in battle without having to take their horses away beforehand, or b) have them give me a horse when mine goes down. (A horse! A horse! My kingdom for a horse!) It sometimes takes them a few moments to do it, and then if they dismount some way from where I've ordered the army to stand, they run back. But oh well.