I’ve encountered a bug in 0.17.147 on Linux where sometimes when placing down monster spawning zones the zones end up being extremely sparse - there are large “bald spots” spanning several “tiles”. Changing spawning density does not affect the bald spots. Alt-tabbing in and out of game will cause the monsters to appear.
I’ve checked the monster count before and after alt-tabbing, and it looks like the actual number of monsters is not affected; the count with and without the bald spot was the exact same, so the bug is likely somewhere within your rendering code.
Here’s some screencaps of the issue.