/etc/network/interfaces entries lost during virtual host addition or deletion

On Ubuntu 14.04.1 and 12.04.2, for the IPv6 interface to function, one has to add the lines:

in /etc/network/interfaces

But Virtualmin 4.11.gpl removes them every time one adds or deletes a virtual host. IPv6 then no longer works until one manually runs the command:

For example, one edits /etc/network/interfaces so the IPv6 section reads like this:

After adding/deleting a virtual host, when Virtualmin adds or removes the allocated IPv6 line of the virtual host, it becomes

I.e. the command ip route add default via fe80::1 dev eth0 is removed even though it was previously manually added.

This appears to be a new bug, because previous versions of Virtualmin did not cause this issue.

It would make more sense if Virtualmin only modified entries it added itself during the creation of a virtual host and left alone any manual modifications in the file made by the administrator.