When adding or removing a domain to Virtualmin, the IPv6 gateway line is removed from /etc/network/interfaces on Debian 8. I originally reported this in https://virtualmin.com/node/37237 along with another IPv6 issue. The problem described there was partially fixed. The gateway line still gets removed when any domain is added or deleted in Virtualmin though. Using Virtualmin 4.18 Pro and Webmin 1.760 on Debian 8.
Adding or removing virtual server with virtual IP deletes IPv6 gateway