[Ryu-Kazuki]

And if it needs any further clarification. To replicate;

Be invited to a party
Be invited to another party while still in the first party (congrats, you're now now in two parties and when either of them move or engage in PvP, you jump between parties; or if like in the screenshots, you can be pulled across the map and your PvP actions work on the main map, hi SL1)
Take over the world

Basically, if you get invited to another party while in a party, it doesn't cut you from the first party. I think it should be so that if you are currently in a party, you shouldn't get party invites unless you leave the initial party first.