It's caused by the so called Benny-Hill code, to prevent players from running light cavalry around the map until the timer runs out.
Ahhh those exploiters cost me a battle. I didn't know this really. Thank you. I was just trying to reorganize my army or whatever but I kept getting undesired formation. Some of them were still engaged so I think that's the problem. I put them all in one group.

About the light-cavalry, they still can run around map when put in skirmish mode. Well I use skirmish method quite often especially if I have access to horse archers. They won't route at all unless they lose men. That makes Benny-Hill code useless, no?