I discovered this while accidentally queueing for celestials in the new premade finder. Right after accepting the invite I greater invisd and left the party, but I stayed on the party leader's realm. No, I was not in combat. I actually couldn't figure out how to get back to my realm for a second. I even ported to shrine and stormpeaks, still on their realm. Only joining or starting another group ported me back to my own realm.
It seems like if you leave during the time it takes to load the realm youre transferring to, then you'll stick on that realm despite not being grouped. Spamming spells during the load time also might help. I'm not having 100% success with replication, but a very high amount. I've been on half a dozen different servers so far and can change zones without changing realm. It has worked with both raids and parties. every group I've joined though has been on timeless isles.
I know there are other methods of manipulating xrealm stuff, hope this isn't a repost.
Edit: after further testing, using the macro /script LeaveParty() and pressing it after my frames from the group popped up but before the people around me phased out helped make this more consistent.