Assume that you use Always On Availability Groups in Microsoft SQL Server. After you switch to serial redo from parallel redo by enabling Trace Flag 3459, serial redo works as expected. However, when you switch back to parallel redo by disabling Trace Flag 3459, parallel redo does not work. If you restart the instance of SQL Server, parallel redo works as expected.
This problem is fixed in the following update for SQL Server:
Microsoft has confirmed that this is a problem in the Microsoft products that are listed in the "Applies to" section.