Thanks again for your understanding whilst we resolved this issue. We apologise for any inconvenience this may have caused.
Upon further investigation, we found the “ipv6” subdomain was not able to be covered by the SSL certificate and was causing it to hang. After removing this subdomain from the AutoSSL coverage the certificate reissued immediately and the domain is now covered by an SSL.
We’re working with the vendor to improve monitoring, in an effort to prevent this occuring in the future.