Do you have a save file where you have some prisoners to ransom? So that we can investigate this issue?
I've created the ticket on
https://www.taleworlds.com. If you need some more explanation from my side or save file to be sent somewhere else, you can feel free to contact to me. My suggestion: prisoners has some chance to be recruited to my army based on characted leadership, rogery and steward skills (it seems and feels correct), but anytime I drop prisoners out and "capture" them back, games rolling the chance again, without memorising previous results.