Added more ways for the Conflict Director to spawn hordes. Hordes can now spawn in different ways to respond to player behaviour, including coordinated strike with Specialists, mixed in Elites with the horde, push from behind while ranged enemies spawn ahead, bigger ambushes with more spread and more.
Patch #13: Class Overhaul Part 1
I believe what’s happening is that the new Horde Spawn variations aren’t properly recognized or used by the Conflict Director correctly.
When the old spawns happen, everything works as intended.
When the new spawns happen, you get a whole lot of nothing, with only ambient enemies around.
Alternatively it could be that the new spawn methods are in conflict with the old.
Whichever it is, this change will be responsible the issue when it crops up.