I ran into an issue where locally, the server was not showing joined to the Availability Group, however but primary still shows it as joined.

Symptoms –

Server is still added to the availability group if you view the primary.

When browsing to the AlwaysOn High Availability from the affected server, it shows up blank

The affected server is still in the failover cluster

The affected server still has the checkbox for being added to the Availability Group (SSCM)

Failed Troubleshooting Steps –

Rebooted the affected server.

Failed over Cluster resource to simulate a restart, no change.

Resolution – 

Sadly, we had to kick the server from the AlwaysOn Availability Group and rejoin the server.

Steps to fix –

• Stop SQL Services on the affected node.
• From the Primary Replica, remove the affected node from the AlwaysOn Availability Group.
• Clear the checkbox for Enable AlwaysOn Availability Groups through SSCM, hit apply.
• Check the checkbox for Enable AlwaysOn Availability Groups through SSCM, hit apply.
• Start SQL services on the affected node.
• At this point, any databases that were in the availability group will need to have T-log restores, or, in my case, need to be dropped and manually re-seeded.
• Once all databases are seeded, verify the health of the AlwaysOn Availability Group via the dashboard.