I'm playing a SP campaign against 7 AI opponents on a large random map. Very early on one of the opponents became listed as being destroyed. Thing is the faction in question still had at least 3 planets and quite a formidable fleet? After getting "killed" they just stood there doing nothing, except defending when I attacked them. The other AI's gave some missions to kill the "dead" factions ships and facilities every now and then as well. Though I couldn't have the pirates attack them anymore.

What's this undead thing? Has anyone encountered it before? Is it just some bug?