That's likely having to do with EMV contactless. Your phone knows the currency even before the transaction hits the network, so it gets "remembered", whereas with US Amex, the phone waits for a push from Amex to know the amount+merchant name because of MSD. Since it shows on the statement as USD, it will be pushed to the phone as USD. This is purely speculation though.

By the way, do non-US Amex cards get notifications for non-Apple Pay transactions in Wallet as well? Does that also show local currency?