Unable to complete Abyss pits

I am constantly running into an issue where I am unable to complete Abyss pits, as there are no monsters to kill to open them. This happens in regular Abyss maps and overrun maps, even when I clear a map to zero monsters left.

The attached image shows two pits I couldn't complete. There should be a trigger or something when the number of monsters on the map reaches a certain number, maybe 50 monsters left or something, so that they always open.