Okay, after doing some experimenting, it seems this glitch was only happening in my swamp furnace? I went to a different furnace and it works fine now! (Also, there seems to be a glitch where monster noises will keep repeating after they're dead? It's only happened with the newest update, though)
My bases certainly won't be happy with the new monster spawn system, but I sure am. Thank you for the update! I will let you know new glitches as I find them!