Part of the problem might be that not all Design Hotels participate, but this one apparently does. There can also be a problem if one were to start on the Marriott website and then click through to the hotel's own website when making a reservation, although it doesn't sound like the OP did this. [Of course, for "regular" Bonvoy properties, if you click into the hotel's website, you're still booking through Marriott and it clearly counts.]