The bug with Rattlers is that sometimes the cluster munitions inside the missile don't get restocked. That's most likely why you see this happening.

I remember when Rattlers worked well 😃 and everyone was spamming them, but that was many years ago. Missiles in general started working better again in 3.18, but they're still pretty hit-or-miss. And NPCs will target and shoot them now! so Rattlers might be the way to go (too many projectiles for them to get them all).

I've been wondering this myself. They seem to work fine sometimes.

My current theories are, the further away fired the more likely they split, might be affected by server tick, could be to do with resupply.