I'm not entirely sure this is much of a bug, but anything that is an entity that ends up in an end portal gateway (Firework rocket, pigs, etc.) Will cause the teleportation of said entity along with the sound from the teleportation. It's likely it's nothing special and wouldn't happen in normal play. Though it does happen so I can't be sure. I've only figured this out by stuffing firework rockets and chickens into gateways for some reason. Just a minor inconvenience, but again, nothing special.