I have 3 IAP-225-US running 8.6.0.2. They used to be joined to a virtual cluster and then randomly dropped out of the cluster. They are on the same switch and can ping each other. They all have the same virtual-controller-key. They also allow auto join of new aps. I have another cluster of the same ap's working without issue.
Edit:
show ap debug cloud-server
IAP mgmt mode :local-mgmt
cloud config recved :FALSE
autojoin mode :disable
state diff :disable
Device Cert status :SUCCESS
But "Auto join mode" is enabled in the gui...