DNS Group not releasing IP(s)

Greetings!   So I'm having an issue with our UTM that I wanted to run by the forums, maybe see if anyone else has seen this or knows of a fix. 

We use the 'DNS Group' Definition to define specific computers which need to use the Agent for authentication (these groups themselves go into a Network group which is tied to it).  The DNS group works fine resolving the IP (or IPs) associated with the DNS name, however the problem is that it doesn't revert to 'unresolved' after the DNS record no longer exists.  After a week, our PCs DNS records will fall off the system, but the UTM will still show it resolving to whatever those IPs were when it was online.  This is causing problems because then other computers will come along and pick up those IPs and the system will think they need to use the Agent and won't let them out, because they don't have the Agent installed (we only use the Agent for our Mac computers).    I have tried adjusting the 'Timeout' settings and the interface binding on the DNS Group record, and nothing makes it give up the IP(s) except to change the Hostname to something else, then change it back.  I even tried flushing the system DNS cache to no avail.

Any thoughts?

Still running 9.5 UTM, anyone know if this issues exists in 9.6?  

Thanks for your time!