They dont split when launched at a player.
They split on drones, slaves and ai just not the player

Yes, I noticed this feature too. I launched a Large MIRV Torpedo at a teammate and it wouldn't spawn the little missiles that it's meant to while it did work ~3days ago on other team mates.

ALSO: As I write this I'm in Primal Rhino and this feature exists here too, the MIRV's don't work here, however they did work in Primal Zebra.

maybe the one you shoot at must be enemy?

It's the same principle as slaves (because, well... they *are* slaves code wise): target either needs to be hostile, or target-able (e.g. squadded / on team etc) in order for them to fire.