Symptoms
When you add an AlwaysOn availability replica to an existing availability group, the setting that you specified for the SEEDING_MODE option is not respected. For example, you set SEEDING_MODE = AUTOMATIC. However, SEEDING_MODE = MANUALĀ is used instead.
Resolution
The issue was first fixed in the following cumulative update of SQL Server:
Each new cumulative update for SQL Server contains all the hotfixes and all the security fixes that were included with the previous cumulative update. We recommend that you download and install the latest cumulative updates for SQL Server:
Status
Microsoft has confirmed that this is a problem in the Microsoft products that are listed in the "Applies to" section.
References
Learn about the terminology that Microsoft uses to describe software updates.