I've had a few bug reports that for example:

prepend dhcp6.name-servers 2001:470:20::2;

in dhclient.conf doesn't work with 'dhclient -6' (unlike 'dhclient -4').

I tried to ask [1] on dhcp-users few years ago, but with no luck.

I'm reporting it as I consider it a bug.
I'm hoping to figure out how it's supposed to be working and will attach
a patch one day.

[1] https://lists.isc.org/pipermail/dhcp-users/2010-October/012497.html

With regards,
Jiri Popelka