Hi Narsimha,
Could the issue be due to the fact that User Group for Authorization is already a field that is provisioned through standard GRC functionality? When you add systems or roles to a request, you can navigate to the "User System Details" tab and there will be a line item for the system(s), for which you can choose the User Group.
Essentially, you are trying to map a custom field to a UME field that already receives this info from the request. Just my thoughts.
Regards,
Ken