I noticed they were doing this when I saw that in-band DNS updates were failing. Eventually I realied that my router (or something upstream of it) was returning

I noticed they were doing this when I saw that in-band DNS updates were failing. Eventually I realied that my router (or something upstream of it) was returning an error to the client rather than passing through the GSS-TSIG-signed nsupdate packet. So far I have been unsuccessful in my attempts to get through to a technician who knows anything other than "try turning it on and off again". I suspect this policy is deliberate.