SSL Site (Lets Encrypt) not working on IPv6

We've got 2 servers as near to identical as possible both running

Webmin version 1.890 Usermin version 1.741 Virtualmin version 6.03 CentOS Linux 7.5.1804

It has been brought to our attention that HTTPS sites on one of the servers were not viewable on one of our clients networks, but on their mobile devices (and for us) they were fine (HTTP sites were fine everywhere)

After much head scratching it appears that the sites aren't displaying when being viewed where the the client has an IPv6 address (although this could be a red herring)

Having checked other HTTPS sites on the same server (using www.ipv6proxy.net) they all seem to have the same issue. The one's on our other server all seem to be fine.

All the sites are using LetsEncrypt certificates.

I've been through as many settings as possible comparing between the two servers and the only thing I can see different is the affected server has the IPv6 in shorthand notation.

Example domain that seems to have the issue is cardealerwebsite.co.uk Example domain that is fine on the other server is surveysunlimited.co.uk

Has anyone experienced anything similar or got any ideas what the cause/fix may be?

Thanks

Kenton