Bug or WAD: Cannot capture slaves while sieging...

[Ballbarian]

Nation:
Mictlan (EA)

Situation:
Deep in enemy territory, my army has suffered huge losses of slave chaff and the plan was to take an enemy province containing a fortress and make a stand. I figured that I would have half of the commanders maintain the siege and the Tribal Kings (730) would capture slaves to reinforce the ranks for catching arrows. To my surprise, when you select "capture slaves", the commanders orders revert back to siege immediately.

Confusion:
Seems strange that I can perform a blood hunt, summon allies, or reanimate undead while a portion of the army maintains the siege, but I can't capture slaves...