Originally Posted by ISTFlyer

Unfortunately, when you enter a QRPC number into your booking, it sticks there forever unless the attached QRPC account is closed and not available anymore.
When entering another airline FFP number, QR is responsible to pay for those miles to the other airlines and this is why when someone benefits from a QRPC perk ( in this case: more flexible destination change ) the miles get automatically credited to QRPC.

By the way, QR is not the only one who does this.
TK does it when you use the free Wi-Fi for Miles&Smiles members, the US3 does it when you use the free checked bag perk of their respective credit card program, etc...