I have seen this as well where a single member of a unit becomes stuck in the terrain.the unit continues to function and the combat indicator becomes active on the unti card. I have seen this in .74 as well as .8.

The unit carries on as directed and the stuck sprite associates still as part of the unit. You can attack the single sprite and it is if you are fighting the whole unit. Most unfortunate if your cavalry charge over the sprite and it is part of a unit of pike!