SEPA branch number validation

The SEPA format validation appears inconsistent to me, sometimes the Branch number missing causes an error sometimes, but not always. On an automatic receipts execution report I have received an IBY40002 error for one customer, but not the other. Both Banks are in the Netherlands so it doesn’t appear to be a country validation difference. We then enter the Branch number and no longer get the error

The validation is Sepa Core DD 3.3 Batch Order-Level Validation. Has anyone any ideas?

Thanks