It’s been some time since the server merge was done and after this process, me and my friends have been experiencing a very annoying bug when trying to create a party. We managed to map what is happening and what to do to make it work for now, but a fix will be needed.
Sometimes a friend added to the friends list loses reference to the server (the server name does not appear), and when in this state, it is not possible to invite them to the party.
To work, it is necessary that these friends stay offline and return to the available one, this way the server name will appear and it will be possible to invite them to the party.
I hope this information is useful for fixing this bug.
I am also providing some screenshots.