I think I found out, why the Iron Citadel never counts a completed

Normally, bosses count as regular monsters, so if you completely clear the rest of the map, there's one monster left and after you kill the boss, that count goes to zero, e.g.:





In the Iron Citadel, however, Count Geonor counts as monster just as usual, but if you kill him, the counter doesn't go down:





There's definitely no monster left on the map, I tried to find the "leftover" in two Iron Citadels and looked in every nook and cranny, but it's nowhere to be found, so it has to be Geonor.