They are meant to last until muta waves. After muta you get some stalkers spawn points outside AL(bet you didn't know that). You just didn't notice because you have worm waves and you camp. Saying "or else I would've found out" isn't a good reasoning. I know all spawns points for mobs and bosses(apart from the mobs that spawn radius away from team. Know the formula just not exact location for those). If the density of them has increased + spawn rate has increased that's different. 4zombies in every 8seconds sounds like the normal spawn rate to me for recruit.