I have had the same issue when using google fetch and render or google's mobile friendly test, and found, through my hosting company, that it only happens when using
and not with non www. But they and I still don't know why.
Maybe there is no canonical url and google thinks there are two different urls (www and non-www). In Joomla there is an easy way to tell google that an url is canonical; just go to plugins and look for "sef" term. You should get the System -SEF plugin. Entering on it if you put the url to your site in the "Site Domain" field every url will have the tag "canonical".
Also the .htaccess protection can add a line to redirect non www queries to www. This way we would have only one entry for every url in google.