bind drops privileges and loses interface

We have configured bind 9.x on an opnsense box.

This DNS server is acting as a resolver for a local zone and as an authoritative server for our networks. It is bound to two interfaces (both VLAN types).

One of the binding only lasts couple of hour until we have a message saying :

06-Jun-2017 00:00:01.127 error: could not listen on UDP socket: permission denied
06-Jun-2017 00:00:01.127 error: creating IPv4 interface igb2_vlan200 failed; interface ignored

If we restart our server it will bind "ok" for couple of hours until interface is dropped once again.

We have been reading couple of threads here and there with mentions of this problem, and suggested action is to run bind as "root". Needless to say that this is not the kind of thing that you want to see (specially with bind which has a long bug tradition behind it).

So I would like to know if there is a proper way of tuning this ?

And how ?