An artist tag with multiple artists is not what you think it is. It is not one tag with multiple artists in it, it is multiple Artist tags each with one artist in it.
As you have seen, this will show up in various software packages in different ways. Mp3tag displays the combination of these multiple tags as double backslashes. But if you were to open the "Extended Tags..." panel for that file in mp3tag, you should see multiple artist tags, each with one artist in it. If that isn't the case, then you have a tagging problem. If it is the case, then you have an Emby problem.
This is why a screenshot of MusicBee would be helpful.