Abyss Holes Not Spawning Monsters (bug ID #531,383,235)

Quite often there will be one or two abyss holes that do not spawn any additional monsters after the initial ones have been killed around it. Most recently, I had three in one map that didn't spawn any additional monsters. See screenshot below with 0 monsters left in a completed map and three highlighted abyss holes indicating they have not triggered spawning additional enemies yet.

Bug reference number: 531,383,235