/*/cac:TaxTotal/cac:TaxSubtotal/cac:TaxCategory[normalize-space(cbc:ID) = 'AE'][cac:TaxScheme/normalize-space(upper-case(cbc:ID))='VAT'][BR-AE-10]-A VAT breakdown (BG-23) with VAT Category code (BT-118) "Reverse charge" shall have a VAT exemption reason code (BT-121), meaning "Reverse charge" or the VAT exemption reason text (BT-120) "Reverse charge" (or the equivalent standard text in another language).
Hitting this rule in production? Paste the invoice into the validator or wire the API into CI so it never ships broken again.