1.
BAPI_ACC_DOCUMENT_POST internally do a call transaction to FB01. You debug your program and put a break point on statement CALL TRANSACTION. Now check the posting in foreground mode. ( Change value to A from N). There look at tax line posting. I think you will get some clue.
2.
Some times extra lines comes in BSET with zero tax amount if there any zero tax rate maintain in FTXP.
3. If you does not get any clue from the other two step, you can try to post the document manually from FB01 with the same data and see the BSET table.
Thanks
Subhankar
