The only thing I can see is that the engine may be confused by all the raiders mixed in with the light inf/dragoons
Certainly the Rangers and Natives cannot capture, but I can't recall if light inf or dragoons can.
Try removing the raiders so that only the legions remain..
I have vague recollecon that a unit/stack assumes the 'character' of the majority model, so perhaps the raiders and dragoons are the issue...
Any nearby militia should seal the deal....