When I have race-wide agents placed in a civilization that gets eliminated, those agents just disappear. It seems like I should get them back.

I just finished a game where I had 4 agents in the Drengin Empire, then had their last planet flip to me via culture. They were eliminated, so of course their agent control box disappeared from my Espionage screen. But the agents still seemed to be active, since a couple turns later I was notified that I'd achieved "Advanced"-level espionage on them. (So they actually don't seem to be vanishing; I just can't remove them, since their box disappears from the Espionage screen.)

Is this a bug, or is it working as intended?