The AlwaysOn Health Events were consistent with the SQL Server Logs. I discovered that the Windows Server Failover Cluster had been down (but was back online), and that the AlwaysOn Group had failed over and back. But the culprit to my secondary replica and databases being disconnected was an endpoint issue. I found in my SQL Server logs the service account running AlwaysOn had for some reason lost its permissions to connect to the endpoint:
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