AutoSSL for add-on domains under single account?

Newbie here. Cannot figure this out. Account in WHM is owned by 'mysite'. There is a domain associated with this account, example.com -- and it has a paid for wildcard SSL. When I create an add-on domain (e.g. domain.com) under this account and then visit the autoSSL facility in cPanel, it tells me that the domain is already covered by a certificate (the one bound to example.com) - and therefore will not be managed by autoSSL. It also says that the certificate does not match the domain name, and the domain is insecure. I somehow end up getting the www.example.com version of the domain to be covered by autoSSL but it's through an arcane process of:

• changing the add-on domain so it now has root path public_html
• deleting the prior root directory public_html/example.com
• running autoSSL TWICE before it works - only for the www version of the add-on domain

The first time it complains that the non-www version of the site could not produce the .well-known/pki-validation file. The second time, that message is still there but then the www version of the site is green under autoSSL, and validated. Any help would be very much appreciated. I'm not confident at all that this solution is going to be robust and later auto-renew when the certs are about to expire.