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This is pretty consistent, I'll fight a siege battle in my current Von Carstein campaign and afterward, I'll check the Raise Dead options, and no matter how many casualties there were, my only recruitment options will be a couple Skel Spears, Swords, and Zombies.
I've noticed the same results if I'm being sieged and win, or if I'm being sieged and an ally attacks the sieging army and I assist. Normal field battles usually work like they used to though.