This issue has been addressed in the following release(s):

Enterprise Vault 12.2
Please follow the software download instructions per article https://www.veritas.com/support/en_US/article.100040083 to obtain the latest release.

Notes:

1. Starting with the EV 12.2.0 release, any new Custodian Group that is manually created and configured to synchronize with an AD group, will accurately reflect any changes subsequently made to that AD group whenever the Custodian Group is synchronized. 
2. DA 12.2.0 and greater do not correct any synchronization errors in any existing Custodian Group that have been manually created and configured with an earlier release of DA.
3. Due to the nature of AD Dynamic Groups not having a set listing of members, Custodian Manager will not synchronize any Custodian Group against an AD Dynamic Group.  AD Dynamic Distribution Group members are determined through LDAP usage.  A Custodian Group can be configured to use the same LDAP query used in an AD Dynamic Distribution Group to obtain the appropriate members.  Refer to Veritas Article 100032660 for more information about obtaining the LDAP query used in an AD Dynamic Distribution Group so that it can be applied to a Custodian Group.
4. Similarly for Compliance Accelerator (CA), due to the nature of AD Dynamic Groups not having a set listing of members, CA Employee Group Synchronization will not synchronize any Employee Group against an AD Dynamic Group.  AD Dynamic Distribution Group members are determined through LDAP usage.  A CA Employee Group can be configured to use the same LDAP query used in an AD Dynamic Distribution Group to obtain the appropriate members.  Refer to Veritas Article 100032660 for more information about obtaining the LDAP query used in an AD Dynamic Distribution Group so that it can be applied to a CA Employee Group.