Reading between the lines, it sounds like the "defective procedure" was not in any way your responsibility.
Was it a kind of random event, or was it down to faulty procedures being used which were the responsibility of the university?
Th fact that the uni waived it to allow you to progress shows that discretion was used and that the university was taking responsibility.
Were you offered a chance to retake the assessment?
Did you think about appealing against the 15% last year?