One more datapoint for this thread.
Made a AUD$3180 payment on the Arrival+ on 12/24. Took a while to post but just did so.
Barclays claims the transaction post date is 12/28. They converted it to $2285.58. Plugging in my values here:
https://www.mastercard.us/en-us/consumers/get-support/convert-currency.html
Gives me that exact value but only if I choose 12/27. Maybe a time zone thing. Unclear which time zone the MasterCard website operates in.
So... maybe something to keep in mind for later comparisons.