This is ongoing, so I can't give a definitive answer.
But just to recap my scenario, I booked a one-way flight for two pax. I don't remember the exact fare but it was ~$370 per pax.
This was a single booking for two pax and was a single transaction on the Amex.
I received one Amex reference number and one QF PNR and a receipt for $740.
However, on my Amex statement, rather than a single charge of $740, instead I had 2x$370 listed.
Thus it never reached the $450 threshold because Amex split the transaction and so far I've received no credit at all.
Update on this. In short: resolved and $450 statement credit manually applied.
Long version:
When I first contacted Amex about this I was questioned extensively about whether I’d correctly selected the “radio button” to use the credit.
I explained (politely) that it wasn’t my first rodeo and that I certainly clicked the radio button. I also told them I had screenshots for every step of the booking process. (My lack of trust was implied, and as it turns out, justified. But I digress.)
The agent still believed the most likely cause was me not clicking the button but escalated to IT for investigation and said it could take a couple of weeks. Didn’t seem interested in my screenshots to confirm.
At about the two week mark I was sent a courtesy email advising it was still being investigated.
What was interesting was that the email included an exchange between the agent I was dealing with and someone in IT. I suspect this was included in error.
But helpfully it showed details of every step I’d taken through the booking - and importantly that I’d clicked the correct button to apply the credit.
After that email I heard nothing further for a couple of weeks then last week saw the credit had posted to the account.
The apology from Amex must have been implied too because I haven’t heard from them since.
So I’m none the wiser about what happened but at least the $450 credit was honoured.
