Alright, great to hear it is intended result, not a bug.
unfortunately, screamers never spawn at all even in towns/cities.. I played with friends couple days on higher tier quest and we've using guns, a lot.
Usually the game spawned screamers when we generate too much heat/noise in POI inside a city. but it never spawned at all.
So the spawned zombies much easier to deal with a lot of guns.
Hope you can improve the mods further, especially on screamer thing, because as server admin, i always tried to surprise my friends to summon a screamer at base to amp the challenge or to test the base defense