Site is accessible without www cname only

I usually set up all sites the same, hosted on SiteGround, Lets Encrypt SSL, Really Simple SSL.

NameCheap for domains, pointing to SG DNS, then migrate to CF.

For some reason I have a site that only will resolve without www:

CF support said the cert was invalid and to change SSL/TLS to Full instead of Full (Strict) . . . when I go to the site without using the cname it works fine and shows a valid cert.

Why would the suggestion be to use a setting for self-signed certificates?

Went to the cpanel/let’s encrypt settings, cancelled the cert and re-issued, all works now.