I play a realm with my nephew.  We are both in the over world.  we have no issues until one of us tries to enter a nether portal, the one who enters gets stuck at a "Building terrain" screen.  The other player can continue moving around but can't interact with the world.  we both have to force quit the software.  Upon restarting, neither of us can join the realm.  To restore functionality i have to restore a backup from before we attempted to enter the portal.  I have done some experimenting.

• The issue occurs in downloaded copies of the realm
• the issue occurs with new portals, and breaking and re-igniting existing portals
• the issue happens on various platforms
• I was able to travel a ways from our base, create a portal, enter the nether and return to the overworld repeatedly, however upon traveling to our nether base and attempting to portal back to the overworld, the game locked up.
• the issue only happens in one of the 3 worlds in the realm
• the issue happens in even the oldest backups, which unfortunately aren't very old at this point
• downloading a copy, and uploading it to replace the world in the realm, made no difference.