Snom D785 3CX Failover not working

I have a system in failover with many snom D713, some GrandStram HT812 and a couple of snom D785.

In case of failover all the phones (except the D785 firmware 10.1.175.0 as supported by 3CX) in a few minutes will register itself to the passive system (correctly became active) and all the services and functionality works great. All the failover process works fine.

The 2 snom D785 never try to register to the new active system (internal DNS change is correct and nslookup told me that the name croceblumodena.my3cx.it has the A record as passive installation).

If I'll go to the web interface of the snom I'll see on log page that the D785 phones try to register to the broken 3CX, the DNS cache page report me that the DNS name will be resolved as old IP and capturing traffic between D785 and DNS servers I'll never see any request done from them. I've leaved that situation for 2 weeks (suspect some TTL problem) and the phones still try to register to the broken system after that time.

If I'll reboot the phones they register correctly to the ACTIVE system.

All the devices are provisioned by 3CX server and are in DHCP, NO MANUAL configuration for no-one. All the phones are on the same subnet.