I added the Parthian Horse Archers from RTR 6.0 to my own mod, and this happens if you zoom out :
The strange thing is, it doesn't happen in every battle, but from time to time.
If you zoom in, to the place where the horses are, everything is fine:
Have you ever experienced something like this? Oh, and I've deactivated sprites, so I think it can't be a sprite problem...