When someone is captured they're typically dragged off the battlefield while the battle continues, no?
There are two possibilities:
1. The capturing unit holds their own prisoners until the battle is over, whereupon all prisoners are transferred to their realm's prison.
OR
2. The capturing unit sends their prisoners to some makeshift holding area for the entire army until the battle is over, whereupon all prisoners are transferred to their realm's prison.
If #1, then when the unit is "wiped out", the prisoners should be set free, because there is no one left in the unit to maintain custody of the prisoners.
If #2, then when the opposing army is defeated, they lose control over all their prisoners. Special Forces should also be given the opportunity to effect a rescue, but that would be a different feature request.
I was explicit in my feature request, but I'll say it again here: this should only count for prisoners captured in the same battle where the capturing unit is later destroyed. Units that are retreating from the battlefield in an organized way are probably not going to lose track of their prisoners.
