A very similar thing happened to me in the 1.2.0 beta branch. I was caught by an enemy lord, and persuaded him to abandon his faction. I paid him a bunch of money to do so. Then after leaving that dialogue and every time after, he did the same thing - acted like he had the first time, like he caught an enemy, but then after that opening message, the dialogue was as if he was an ally. I would then close the dialogue and try to move away, but he would immediately start it all over again. I eventually had to reload an older save to get around this.
If it would help the devs I also have a full video of it happening.