If you use BAPI_ACC_DOCUMENT_POST to create FI Document, you will definitely get the header text is not posted although you have input header text into BAPI_ACC_DOCUMENT_POST.
The solution you need to use BADI AC_DOCUMENT of method CHANGE_INITIAL & CHANGE_AFTER_CHECK.
Go to SE18 and enter AC_DOCUMENT into BadI Name field.
Create enhancement and put this ABAP code into method CHANGE_INITIAL & CHANGE_AFTER_CHECK.
DATA: wa_header TYPE acchd.
IF sy-xprog NE 'SAPMSSY1'.
wa_header = im_document-header.
ex_document-header-bktxt = wa_header-bktxt.