It sounds like a "feature". ;D
When you're making a mission in this game you regularly hit the buffers like this. The trick is to roll sideways: think laterally. Put all your assumptions about how this mission is going to turn out to one side for a moment, and create a totally open mind.
There are lots of ways of bashing this one on the head: for example reveal commands followed by doTarget and doFire.
Or you can say, hey, so it seems these guys are gonna get away with stealing the wagon. Well, good luck to them. What does that mean for the mission? How do I re-create the next part to take account of this?
Maybe there is a roadblock - I mean with the road genuinely blocked - round the corner. Or perhaps the road is mined. Or maybe it doesn't really matter that they have wheels because they're gonna have to get out and walk sooner or later. Or maybe the wagon breaks down. OR maybe you don't really need the vehicle there in the first place. Or maybe it could be much better guarded. Or - well you get the picture.