Hello!
I have solved the same issue with implicit Enhancement in include LMRMDF0H, form t_drseg_aufbauen_pruefen_1, at the end of form code I cleared position tax code in a loop:
| ENHANCEMENT 1 ZMUK_MIRO_TAXCODE_EDIT. | "active version |
FIELD-SYMBOLS <ls_drseg> LIKE LINE OF t_drseg[].
LOOP AT t_drseg ASSIGNING <ls_drseg>.
| CLEAR <ls_drseg>-mwskz. |
ENDLOOP.
ENDENHANCEMENT.
After that system is propagating tax code automatically from MIRO header to all the items.
You can see also BADI EXTENSION_US_TAXES.