Not able to populate data from nested CDT on grid

Hello,

I have array of CDT within CDT. When I'm trying to populate data from that array to grid column I'm getting blank value with index function.

CDT Structure:

Datasubset value:[ userAssignedRequestName="test",
      users=[userName=dc_creator, email=dc_creator@bankcorp.com, phone=410-858-8811, firstName=test User, lastName=, status=active, role=xxxx, lastUserAction=creator];
            [userName=dc_modifier, email=dc_modifier@bankcorp.com, phone=410-858-8812, firstName=test User2, lastName=, status=active, role=xxxx, lastUserAction=modifier]]; 

Below code is not returning user array:

  a!gridTextColumn(
                  label: "User Name",
                  field: "lastUpdatedUserId",
                  data: rule!REST_getLastUpdatedUser(
                    index(                           
                              local!datasubset.data,
                             "users",
                             {}
                           )                  
                        )
                   ),

above highlighted code is not returning users array. Can anyone faced this issue with nested CDT ?

inside rule rule!REST_getLastUpdatedUser I have logic to return particular user name.