Comment on WordPress Site Title Tag SEO 2016 by SEO Dave.
As you can see on the Google datacenter I’m using (google.com so whatever they are serving me by default) they aren’t showing the name of the site.
What will be happening is when you don’t have the name of the site as part of the title tag the Google datacenter you are on is adding the name of the site. When you add the name of the site via your title element the Google datacenter you are in knows this (after a reindex) and so doesn’t add it to the search result.
What Google is showing on different datacenters will have no impact on your SERPs, they’ll be testing the user experience to see what their users prefer.
If you look up the different Google datacenters you’ll be able to do a search on different ones and get different results. So you should be able to find a datacenter that does show the name of the site and others that don’t.