Minimum requirements for Active Directory Failover to work are described here:
https://techdocs.broadcom.com/us/en/symantec-security-software/identity-security/identity-management-and-governance-connectors/1-0/connectors/microsoft-connectors/microsoft-active-directory-exchange-and-skpye-for-business(lync)/Managing-the-Connection-to-Active-Directory/active-directory-failover-setup.html
Release : 14.3, 14.4
ADS_Failover and ADS_RETRY will only take place if one of the below errors happens:
LDAP_SERVER_DOWN
LDAP_UNAVAILABLE
LDAP_UNAVAILABLE_CRIT_EXTENSION
LDAP_TIMEOUT
LDAP_LOCAL_ERROR
Other LDAP errors should not cause failover and will need to be investigated.