If people have asked a BA agent at the airport to do it and it hasn’t worked then that person hasn’t made the change properly.
Get them to reprint your boarding pass and check that the new number is on there.

Other than ‘previous’ issues, I haven’t seen any recent data points that suggest this is actually an issue.

Slightly different but I had my BA Gold in a CX booking. I checked in and then asked the agent to change it to QR, which they did. QRPC Avios and Q Points were in my account within 72 hrs and I had no issue entering the CX Pier F in HKG.