How to populate custom field for app Manage Customer Line Items ?

I'm trying to populate a custom field in the Manage Customer Line Items app. I've redefined method ITEMSET_GET_ENTITYSET to loop over the entity set to update the custom field, but the changes are not reflected in the Fiori App where the custom field remains blank. However, updating any standard field (like ASSIGNMENTREFERENCE) does work.

Here are the steps I've followed so far:

Create APPEND structure to FAR_IFIARLINEITEM_EX containing custom field ZZTXT.

Create APPEND structure to INCL_EEW_FAR_CLI_ITEM_MD containing custom field ZZTXT.

Create SEGW project ZFAR_CUSTOMER_LINE_ITEMS and redefine data model of OData service (SAP GW) FAR_CUSTOMER_LINE_ITEMS.

Add a custom property ZZTXT to the "Item" entity type, ABAP field matching the custom field. "Creatable" "updatable" "sortable" "filterable" and "nullable" are unchecked.

Redefine ZCL_ZFAR_CUSTOMER_L_01_DPC_EXT~ITEMSET_GET_ENTITYSET to call the super method, loop over the entityset and update the ZZTXT.

Activate the project in SEGW (without errors).

This is the code in the redefined ITEMSET_GET_ENTITYSET method:

 method ITEMSET_GET_ENTITYSET.

CALL METHOD super->itemset_get_entityset
EXPORTING
  iv_entity_name = iv_entity_name
  iv_entity_set_name = iv_entity_set_name
  iv_source_name = iv_source_name
  it_filter_select_options = it_filter_select_options
  is_paging = is_paging
  it_key_tab = it_key_tab
  it_navigation_path = it_navigation_path
  it_order = it_order
  iv_filter_string = iv_filter_string
  iv_search_string =  iv_search_string
  io_tech_request_context = io_tech_request_context
IMPORTING
  et_entityset = et_entityset
  es_response_context = es_response_context.
*  loop at result set and calculate the new field(s)
LOOP AT et_entityset ASSIGNING FIELD-SYMBOL(<fs_es>).
  <fs_es>-zztxt = 'TEST 1234'.
  <fs_es>-assignmentreference = 'ABABA'.
ENDLOOP.
endmethod.