Command by Mail and custom fields

Using RT 3.8.8, with RT Extension command by mail active. Two
privileged users belonging to the same custom group. That custom group
has rights to modify, assign and see custom fields to tickets in Queue
xyz. Both users were unable to assign custom field values, so I
modified the Configuration->Global->User Rights and granted User A,
Assign, Modify and see Custom fields permissions. Once that was done
User A is able to update basic and custom fields to tickets within queue
xyz through email. Granted the same to User B in
Configuration->Global->User Rights, but User B still cannot assign
custom field values, but is able to assign basic field values, through

Please provide list of permissions to grant so User B or another
privileged User to be able to assign custom field values through e-mail.

Thank you,
Kamber Dalal