Rancher HA is flapping

We have a Rancher v1.1.0 cluster stood up and barely documented by a colleague who left the company.

Trying to look into upgrading the cluster, so we are standing up a duplicate cluster to test out the upgrade.

Problem is, I have got to the point where we are trying to get HA to work, but the servers flap in and out of HA status.

Does anyone have any suggestions?