T27G - Fall Back issues - Successive setting [Still an issue]

[BobBluePackets]

We are working with two FreePBX systems - configuration is identical on the two, at least in terms of FreePBX/Asterisk base settings and Extensions settings - basically a poor mans redundancy

All phones are on the following firmware, 69.84.0.10, but we have also tested a few of the phones on the latest 69.84.0.15 firmware (just in case).

The phones are auto provisioned, and we have setup the phones to work in fallback mode e.g.
1) SIP Server 1 and SIP Server 2 set (ip addresses for the moment - no DNS names used) - keeping it simple
2) No Proxy setup
3) Registration type set to 1 (successive) - not concurrent

Approximately 140 of 151 Yealink T27G phones Register with the Primary Server correctly and do not register with the Secondary Server (exactly what is expected)
Remove the Primary Server, and they all switch over to and register with the Secondary Server, bring the Primary Server online and within a short time, they are all registered back on the Primary server and deregistered with the Secondary Server

However I have 11 -13 of the T27G models that when brought online, they register to both Primary and Secondary (almost as though they are set to concurrent registration).

I have checked configuration, exporting the configuration from a unit that works correctly and one that does not - performed a diff and found no differences other than the password and extension number details.

Anyone else come across this issue....any thoughts....

Regards
Bob