The way that your MiaB did this is the way that it is supposed to do this.
I suspect that when you set up your first MiaB installation, your domain name was a more standard second level domain as there is absolutely nothing wrong with how this one is set up.
Keep in mind that the nature of your domain having a third level domain may be the reason for this.
Again, this is perfectly normal.
Your MiaB will not function properly if it does not serve the subdomain which includes the hostname. You do not have to use any email addresses with the fourth level domain name if you do not wish to publicly.