It should work. After hours of trial and error, here is what I finally figured out.

This JSON "breaks" and causes the variable name instead of data to passed in for all variables:

{
    "accountIdPath": "Entitlement.message.id",
      "call": [
        {
          "name": "Entitlement",
          "connection": "acctAuth",
          "url": "https://scim.zscalertwo.net/REMOVED/scim/Users",
          "httpMethod": "POST",
          "httpParams": "{\"schemas\":[\"urn:ietf:params:scim:schemas:core:2.0:User\",\"urn:ietf:params:scim:schemas:extension:enterprise:2.0:User\"],\"userName\":\"${user.systemUserName}@DOMAIN.COM\",\"displayName\":\"${user.displayname}\",\"urn:ietf:params:scim:schemas:extension:enterprise:2.0:User\" : {\"department\":\"${user.departmentname}\"}}",
          "httpHeaders": {
            "Authorization": "${access_token}",
            "Accept": "application/json"      },
          "httpContentType": "application/json",
          "successResponses": {
            "statusCode": [
              201
            ]
          }
        }
      ]
    }

This JSON works and all variable data is passed correctly:

{
    "accountIdPath": "Entitlement.message.id",
      "call": [
        {
          "name": "Entitlement",
          "connection": "acctAuth",
          "url": "https://scim.zscalertwo.net/REMOVED/scim/Users",
          "httpMethod": "POST",
          "httpParams": "{\"schemas\":[\"urn:ietf:params:scim:schemas:core:2.0:User\",\"urn:ietf:params:scim:schemas:extension:enterprise:2.0:User\"],\"userName\":\"${user.systemUserName}@DOMAIN.COM\",\"displayName\":\"${user.displayname}\",\"urn:ietf:params:scim:schemas:extension:enterprise:2.0:User\" : {\"department\":\"${if(user.departmentname != null){user.departmentname} else{''}}\"}}",
          "httpHeaders": {
            "Authorization": "${access_token}",
            "Accept": "application/json"      },
          "httpContentType": "application/json",
          "successResponses": {
            "statusCode": [
              201
            ]
          }
        }
      ]
    }

Do you see the difference? Just a little bit of unnecessary javascript using the *same* Saviynt variable.. and this works.

{\"department\":\"${if(user.departmentname != null){user.departmentname} else{''}}\"}

Seems like a bug!