When your using local DNS and requesting a LetsEncrypt SSL certificate for a domain it doesn't seem to include the www. alias - seems as you can't add that yourself as a separate domain, it should also include that otherwise if not using cloud flare or similar there will be a common name mismatch and it might come up with warnings in the users browser when using www (e.g. Nord Threat Protection).
This is only an issue when using the local DNS and pointing a domain to the local nameservers - doesn't matter when using CloudFlare because their certificate covers it.