Consider the following scenario:
You have an Always On availability group with hundreds of databases in Microsoft SQL Server 2014 or 2016.
You are running heavy workloads in this availability group.
A failover of the availability group occurs.
In this case, the databases in the availability group at the new primary replica may show as Not Synchronizing due to long undo operations, and the databases in the availability group at the new secondary replica may show as Initializing/Synchronizing after the failover due to message handling between replicas. Additionally, error messages that resemble the following are logged in the SQL Server error log:
Error: 35278, Severity: 17, State: 4.
Availability database database_name, which is in the secondary role, is being restarted to resynchronize with the ResyncWithPrimary - Damaged  - EOL: 00000047:00000F58:0001 Recovery: FFFFFFFF:FFFFFFFF:0001 Undo: 00000047:00000F70:005F current primary database. This is an informational message only. No user action is required.
ResyncWithPrimary - No Undo Needed  - EOL: 00000047:00000F58:0001 Recovery: FFFFFFFF:FFFFFFFF:0001 Undo: 00000047:00000F70:005F
This issue is fixed in the following cumulative updates for SQL Server:
Microsoft has confirmed that this is a problem in the Microsoft products that are listed in the "Applies to" section.
Learn about the terminology Microsoft uses to describe software updates.