CALL TRANSACTION ... UPDATE 'S' and Restartable Update Request

Question

Dear experts,
I have a program that calls a transaction by using the CALL TRANSACTION ... UPDATE 'S' statement. This transaction then performs a database update by using CALL FUNCTION ... IN UPDATE TASK statement. The database update may cause an error due to the primary key conflict.
One behavior that I notice of my program is when the database update raises an error, the corresponding update request in the update table log (SM13) will be flagged as error and non-restartable, eventhough the update module in question is restartable. Why is it like this?
To make my question clearer, I attach my transaction, program and update module below. They are very simple.
Update module:
FUNCTION z_121_15_update_module.
*"*"Update Function Module:
*"*"Local Interface:
*"  IMPORTING
*"     VALUE(IMP_ZSCARR_CUSTOM) TYPE  ZSCARR_CUSTOM
  INSERT into zscarr_custom values imp_zscarr_custom.
  IF sy-subrc NE 0.
    WRITE sy-subrc TO subrc.
    MESSAGE subrc TYPE 'A'.
  ENDIF.
ENDFUNCTION.
Subroutine in the transaction that calls the update module above:
FORM save.
  zscarr_custom-carrname = sy-uzeit.
  CALL FUNCTION 'Z_121_15_UPDATE_MODULE' IN UPDATE TASK
    EXPORTING
      imp_zscarr_custom = zscarr_custom.
  COMMIT WORK.
  LEAVE TO SCREEN 0.
ENDFORM.
The program that calls the transaction whose the subroutine above:
*& Report  Z_121_15_COMPLEX_LUW_19
*& Program that calls a transaction.
*& Called transaction(s): Z_121_15_CLUW_20
REPORT z_121_15_complex_luw_19.
DATA:
  it_bdcdata TYPE TABLE OF bdcdata,
  wa_bdcdata TYPE bdcdata,
  subrc TYPE char10.
wa_bdcdata-program = 'Z_121_15_COMPLEX_LUW_20'.
wa_bdcdata-dynpro = '100'.
wa_bdcdata-dynbegin = 'X'.
APPEND wa_bdcdata TO it_bdcdata.
CLEAR wa_bdcdata.
wa_bdcdata-fnam = 'ZSCARR_CUSTOM-CARRID'.
wa_bdcdata-fval = '01'.
APPEND wa_bdcdata TO it_bdcdata.
CLEAR wa_bdcdata.
wa_bdcdata-fnam = 'BDC_OKCODE'.
wa_bdcdata-fval = 'SAVE'.
APPEND wa_bdcdata TO it_bdcdata.
CALL TRANSACTION 'Z_121_15_CLUW_20' USING it_bdcdata MODE 'E' UPDATE 'A'.
*IF sy-subrc > 0.
  WRITE sy-subrc TO subrc.
  MESSAGE subrc TYPE 'S'.
*ENDIF.
Thanks in advance.
Regards,
Haris

Answer 1

What I mean is, for example, when the called update module in the code below issues a type A message,
  CALL FUNCTION 'Z_121_15_UPDATE_MODULE' IN UPDATE TASK
    EXPORTING
      imp_zscarr_custom = zscarr_custom.
  COMMIT WORK AND WAIT.
then, in SM13, the corresponding update task will be flagged as non restart-able, eventhough the update module itself is defined as restart-able.
This also happens in the case follows: let the code below is a part of a transaction program that calls an update module
  CALL FUNCTION 'Z_121_15_UPDATE_MODULE' IN UPDATE TASK
    EXPORTING
      imp_zscarr_custom = zscarr_custom.
  COMMIT WORK.
and the transaction above is called by a program by using
CALL TRANSACTION 'THE_TRANSACTION_ABOVE' ....... UPDATE 'S'.
Am I correct? This is my conclusion so far.