/ubl:Invoice | /cn:CreditNote[BR-AG-03]-An Invoice that contains a Document level allowance (BG-20) where the Document level allowance VAT category code (BT-95) is "IPSI" shall contain the Seller VAT Identifier (BT-31), the Seller Tax registration identifier (BT-32) and/or the Seller tax representative VAT identifier (BT-63).
Hitting this rule in production? Paste the invoice into the validator or wire the API into CI so it never ships broken again.