Guess I should add that the army is _not_ remaining with Nosmina, it is just the champion. Also, immediately after the fight, Nosmina has 1 HP, but does within a few turns heal back up to full 36 HP he started with.

I've killed a few Empire heroes who subsequently appeared on the map, but were unselectable.

Yeah, same thing, same champion. Killed her quite a few times, my new strategy is to wipe out all the Empire players before they can hire her!

I had same issue as OP.  It's not the old 'ghost' thing - Nosima was running around after I killed her, trashing my nodes & outposts.  After killing her many times in combat, I finally killed her with an overland spell -- that worked.

Ghost shield is the one that heals you for 1 health when you are hit right? My Sov had the same item when he refused to die and I am almost positive it is the cause.

Edit: The bug probably works something like. You get hit and go down to 0 health, battle ends BUT you get healed 1 health so you don't die.