I am pretty sure this is a hack or exploit, as I had this happen in a game recently. Obviously if you kill them they return to the other side, but when it happened the first time I thought it was a visual glitch on the death screen. No more kills after I respawn so kept going about the game until suddenly we have another invade and the 1st one comes out of hiding for a double invade. Best way to counter it is to be sure to hunt them down instead of hiding, and report the issue to Bungie when you see them still on the field.