Iron Citadel Completion Bug

I have had twice now where I have found an Iron Citadel and killed Geonar before the counter for monsters left starts to countdown and only says "More than 50 monsters remain". When this happens, it does not count as killing Geonar, even though I have been able to loot him. I would then go and clear the rest of the map, and would get stuck with 1 monster left and never get completion credit. I went through the map 4 times after this, combing every corner and no monsters were left.

When I kill Geonar with < 50 monsters left, this does not occur and I get proper credit.

It's a known bug, sadly GGG has done nothing about it (what a surprise)