[CentOS] CentOS 7, NM, and IPv6

Hi, folks,

   I and the other admin here have just been assigned a mission... here's
what's happening: only very recently - the last week? our CentOS 7
boxes, or at least some of them, will lose their IPv6 addresses, and
not get it back.

   1. We're running dibbler on the same box that serves DHCP.
   2. It's been working for many months.
   3. The lease file remains in place.
   4. It appears to try, several times, and then give up - as our
       manager puts it, "I to renew the lease", "Here it is","Nope,
       don't like that, try again", and eventually, after 4 or 5 or
       so tries, gives up.

   One very show-stopping result of this is that NFS starts timing out.

   So: has anyone else seen this behaviour recently, and does anyone have
some idea of what might be going on here?

       mark