I know, it is against the RFCs, but radvd itself allows it not to advertise DNS Servers. There can be some conditions where such a behaviour is desired. I just found out, that pfsense already implemented this:
https://redmine.pfsense.org/issues/9302
https://github.com/pfsense/pfsense/commit/e26ad76e8fd9b947f2ad97195d2153da14d8c594
It would be great to have this as well in opnsene!
If you do not plan to add that feature: Is it possible to maintain an self written radvd.conf without getting it overwritten by the system?
Maybe better add this one via github but I hvent search yet if there already was such a question.
Ok, will open an issue :)