Hello
Maybe the production order is not relevant for MRP, but the reservations for the components are certainly relevant to MRP, otherwise, a purchase requisition would not have been created in the first place. In my opinion, system is working as designed and if the purchase requisition is not changed by MRP, you would end up with a shortage.
One tip for you: when an MRP element is not relevant for MRP, the field "available quantity" should be empty for this element on MD04.
Regards
Caetano