I am currently testing playing multiplayer, without a realm.

As the host, when I travel through a Nether Portal, I arrive at my destination. I can then travel back, through the same portal to my destination realm. At that point, if I travel back through the portal, it crashes the game, to desktop.

I have also just tested having my wife, who is connected to me, do it. She travelled from the nether realm to her home, then back to the nether realm. Upon using the portal a third time to go home, it crashed ME. There's definitely some illegal operation being performed the third time, regardless of who is doing it.

I've marked this private as it could potentially be used as an exploit to crash realms if this works the same there.