F5 gtm last resort pool DNS Record problem

Hello,

Big Ip v 17.1.X

I've found that when there is a GTM last Resort DNS Respone, and the DNS clients asks to a Microsoft DNS Server, Microsoft DNS server gives a NXDOMAIN response ( once every 3 ) .

If the DNS Client always asks directly to F5 DNS there is no problem.

I'm trying to figure out why.

Graziano