Please note that we've updated the Mount & Blade II: Bannerlord save file system which requires you to take certain steps in order for your save files to be compatible with e1.7.1 and any later updates. You can find the instructions here.
@Lord Mannerheim Does the Encyclopedia show that they are in a a town/castle and when you go there, they are no where to be found and the encyclopedia doesn't changes? I am also having this problem and its not limited to one faction. Also this issue happened to be before the beta update, several times but it was with wanderers and they would usually move after a while and that would fix it, but the lord isn't showing up even after a year.
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.