Hi,
During invoicing (MIRO), tax code/s for material/materials will flow from Purchase Order only.
If more then one tax codesavailable in purchase order items,then you need remove default tax code
in MIRO header (as tax code set for default entry in MIRO) and keep it as BLANK, just select
check box of "Calculate tax" and let all tax codes are there item level which are flows from Purchase Order,so now system will calculate tax rate for each line item with tax code as you have different tax codes in line items and save invoice document.
Regards,
Biju K