If there are any links to the non-www version of the URL from _anywhere_ then it can result in the non-canonical URL being indexed.

Also check that you have:

 - the correct `rel="canonical"` link element in your pages (and/or `Link` HTTP response header).

 - the correct URL stated in your XML sitemaps, RSS feeds, etc.

 - the correct preference set in Google Search Console: www vs non-www

> Now that I have the redirect to www in place, this is no longer problematic

Although you do still need to make sure you have no _internal_ links to the non-canonical URL.