Assume that you have an Always On Availability Group (AG) that contains many databases in an instance of Microsoft SQL Server 2017. After you restart the hosted system or the SQL Server Service, some databases on the secondary replica display the "SYNCHRONIZING" status when you view sys.dm_hadr_database_replica_states output from the secondary replica. However, the same databases on the secondary replica display "NOT SYNCHRONIZING" when you view the Dynamic Management Views (DMV) output from the primary replica.
Note If you stop and then restart the mirroring endpoint on the secondary replica, the databases' status change to "SYNCHRONIZING" on the secondary replica.
Microsoft has confirmed that this is a problem in the Microsoft products that are listed in the "Applies to" section.