PMP450 SM not connecting to cnMaestro

I have an issue with PMP450 SMs losing connectivity to cnMaestro and showing as offline. (I'm posting this on both forums as it is either an SM bug or a cnMaestro client bug)

The cnMaestro (self-hosted) IP address keeps appearing in the SM ARP table with an MAC of 00:00:00:00:00:00 and disappearing. This behaviour is visible during cnMaestro connection attempts and ICMP tests from both sides (cnMaestro and SM). cnMaestro is not in the same broadcast domain as the SM.

The SM is bridged, and all other SMs on the same BHS, as well as the BHS itself can reach cnMaestro.

This issue is usually resloved by a reboot of the SM, though today I have a case where it was not resolved by an earlier reboot (cnMaestro shows it offline for >16 days). I believe this may be because post reboot the SM was not associated with the BHS for ~2 hours due to a color code mismatch.

The issue has occured on multiple firmwares, currently happening on, 16.1.1 and though has happened on 15.x firmwares previously.

Is this happening with all 450 SMs or only some of them?  Is there a firewall between cnMaestro and the SMs?  What happens if the SMs are in the same broadcast domain as the cnMaestro server - do they all connect? 

This only happens on some SMs, most connect with no issues.

It is a routed network beyond local bridge groups eg SM<->AP<->Router is a bridge and all traffic beyond this is routed. Behind the SM there is a client router which establishes a PPPoE session with the Router for public connectivity which works fine.

The issue appears to be the SM looking for an ARP on a non local IP address rather than using it's gateway.

I cannot test with cnMaestro in broadcast domain as that is not network topology.