Is it a bug or is it intended?

I've had it happen to me twice so far. I have a huge army. I successfully siege a castle/town. But I fall during the battle. The defenders retreat to the castle. I send in 15 men or so as I'm not there. If they lose, the whole army disappears and I get imprisoned. It happened to me with 2000+ army yesterday. I'm not sure if this is intended or if it's a bug.