As near as I can tell, I have captured a world and its leader. The leader had my naming scheme, but I couldn't reassign him. It was a nothing world with 100 infrastructure and four garrisons on it, but the conquered population had been assigned a leader with a 50% factory bonus. I'd never heard of this leader, he didn't show up in any search lists, and he vanished on reassignment.
Too bad.
This is a bug - he's the alien governor, and his status should have been changed from "Governor of colony" to "Captured". Even though he seems to have vanished, your DB may still be messed up (i.e. he might still think he's governor), so if you see weird errors from the NPR in the future, it could be due to this.
I'm pretty sure I logged this in one of the official bug threads - I'd recommend you log a "me too", especially if you're using 5.52.
John