Hi mallikha,
Database mirroring failover may cause data loss, but I don't heard that it will cause the DBA group to lose sysadmin permissions.
Try to view the default Trace to find the security changes in your database. Please refer to this
article.
If you cannot get any information through default trace, re-grant the DBA group sysadmin permissions, then create DDL triggers or SQL Server Audit to track security changes, Wait until the problem occurs next time, and then analyze it.Please refer to this
article.
Best Regards,
Cris
MSDN Community Support
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