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.

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.