Couchbase subdocument operation

I am trying to update the array/list of values into the  couchbase document with ‘Allow Sub-Document Writes’ checked. The array value replaces the existing value on the CB instead of just appending them into the existing values already present on the CB document.

My parameter in SDC,

Sub-Document Path = tracking_numbers

Sub-Document operation = ARRAY_APPEND("tracking_numbers.USPS", record:value('USPS'))

Incoming data to CB destination is “USPS: {LIST[]}

CB doc structure,

->document 

     ->tracking_numbers:

             → USPS(array):

                      [0]: 123

                      [1]: 456