When in the service of a lord (as a retainer), and that lord is both the Marshall of the realm and camp commander during a siege, the accompanying allied lords and their troops do not join in the assault, meaning that only a small fraction of the available troops are committed to the battle: resulting in an illogical defeat.

Reloading from a previous save might well enable an alternate lord to be assigned as camp commander, but this precludes the liege lord from obtaining a garrison location of their own.