It seems you are unable to join an alternate upload of the same world project until you restart the client, even though those two uploads are of course under different blueprint IDs. Joining the second upload after having been in the first one will cause the world to be downloaded, but you'll be kicked back to your home world. Logs contain messages about "The AssetBundle '...' can't be loaded because another AssetBundle with the same files is already loaded."
I have prepared two test worlds. They are from the same Unity project. Built and uploaded BugTest 1 with the green plane, changed the blueprint ID, made the plane blue (so you can tell which copy you're in), then uploaded BugTest 2.
This issue does not happen on the current non-beta client.