OnValidate trigger doesn't contain the IsHandled - that why
It's hot issue and we use BC15 onperm I can't wait for MS :)
but I found other solution , in OnBeforeValidate I assign
xRec."VAT Registration No." := Rec."VAT Registration No.";
and in standard validation this line of code
if "VAT Registration No." = Customer."VAT Registration No." then
exit;
let me skip the validation