I just downloaded and did a clean install of Manjaro Budgie 18.1. During the installation I chose both a password AND automatic login.
When the installation finished I was prompted with a login screen, but my password didn't work. After I shut down Manjaro and turned on my computer again I was automatically logged in and my password works everywhere.
Obviously I don't have an issue anymore, but I think this bug could be very confusing for new users and should be investigated.