Rare Skull Labels not Appearing if Map has <200 Total Monsters

Yesterday, I was running a Crypt map with zero mechanics on it. At no point did I ever see the yellow skulls to indicate remaining rare monsters.

Because this map wasn't juiced, nor did it possess any additional content, I feel like I low-rolled the amount of monsters the map spawned, and that it began with fewer than 200 monsters.

This is the only time I've ever seen this bug. I full-cleared the map and did receive completion credit without issue. My supposition is that the game's script has a condition that once the map's monster count falls from 200 to 199 via a player killing a monster, it triggers the skull to appear.

Because the map began with fewer than 200 monsters, this condition was never met.