Adding third LTM to cluster - GTM will only see 2 of the 3 devices.

Hi All - pulling hair out with what should be a simple GTM to LTM cluster set up!! Any pointers appreciated!

We have recently expanded our v11.3.0 LTM Cluster to a third unit. Each LTM has a traffic-group for which it is primary for.

We have GTMs (Different units to the LTMs) that query the LTMs.

On the GTM config, there is a Server set up for the LTM cluster - this is set up as a redundant BIP-IP system. Previously this had one of the LTM listed in the Address List and the other in the Peer Address List

When we added the 3rd unit to the Peer Address list, it successfully gets added, however then after a short period, maybe 10 seconds, it disappears from the list.

And the GTM, can auto-discover Virts which are primary on the new traffic group on the LTM, but will mark them as red (depite them being green on the LTM)

So we added the IP of the "New" LTM into the Address list - it fixed the problem and those Virts changed to green, however this bumped the previous address out of the Peer List, and the new one appeared there. (even though we added it to the Address list...??)

Now GTM marks Virts that are primary on that traffic group as down. (ie the LTM that was bumped off the list)

What is the correct way to add a Clustered LTM system into GTM?! Thanks!!