Wanted to share my experiment here, and was gonna wait for the final results but if anyone else wants to give it a whirl:
I made an MPX purchase for a restaurant and used my new Amex Gold and it's pending...as Travel - Airline (shows on the app, not online). So I might get 4x restaurant, OR 3x airline OR 1x everywhere else. It hasn't posted yet so I don't have any confirmation. I'll follow up once I have more definitive information. Just thought it was interesting.
I'm pretty sure it'll code as only 1x.
MPX never did pass through merchant category coding on Amex cards.
(MPX recently stopped pass through coding on Chase Sapphire and Ink cards too, with several DPs on reddit saying AirBnB GCs, which previously triggered the 3x travel categories on the CSR and CIP, earned only 1x.)
I purchased a $10 Starbucks GC on MPX recently using my Amex Plat,* and it posted as "UNITED AIRLINES 1-800-421-4655 TX"; click for details and you can see that the merchant is "MILEAGEPLUS X," the transaction is categorized as a "Special Service Ticket" with Passenger as "LAST NAME /MILEAGEPLUS X".
By comparison, an actual ticket purchased from United posts as "UNITED AIRLINES HOUSTON TX," with the merchant listed as "UNITED ELEC TICKETNG," categorized as "Flight on October 8" and Passenger as LAST NAME/FIRST NAME.
I have these two transactions on my most recently Amex Plat statement, and the MILEAGEPLUS X transaction earned only 1x, but the UNITED ELEC TICKETNG transaction earned 5x. Amex seems to have figured out a way to weed out MPX transactions from earning 5x. I expect MPX to work the same way with the Amex Gold.
*I could have bought a Starbucks GC on Gyft, OD/OM/Staples, Lowe's, through RMN offers, etc., but I was in a pinch and decided to take a small opportunity loss to put some spend on my Amex cards. There's also a small part of me that hopes MPX may start coding 5x again one day, as improbable as that is.