Restrictions after you failover to an upgraded server instance
After failing over from an earlier server instance to a SQL Server 2016 server instance, the database session is suspended.


Once you failover to the 2016 instance, the database is now upgraded fully to 2016. This is a one way trip and will no longer be able to be hosted on anything but SQL Server 2016. Since the partner is not 2016, the data movement will be suspended (stopped). No new data will "flow" back to the older instance.


  It cannot be resumed until the other partner has been upgraded


Until the old principal server (now the mirror server) is upgraded to 2016 you will not be able to resume data movement - no data will flow until you upgrade the older instance to 2016 and manually resume the data movement.