Forwarding this issue to the QA team for further investigation. Thanks for reporting. In the meantime, can you share your save files? You can upload it at upload.taleworlds.com. Please paste the link to this thread in the description box on the upload website so I can identify it.
I can confirm this issue. I have taken the quest to capture a Lord and when ever i fight and catch the lord he says the lines he did before the battle and then leaves. The option to capture doesn't show up.
I'm having the same issue. Also, I've noticed that every time I defeat my intended target, the game says he's in Ain Baliq for a day or so, which I'm assuming is the home territory of the quest giver. I'm playing on e1.50, with no mods. Here's a copy of my save file. I've uploaded it to my Google Drive, since I don't see a way to attach files here.