I can confirm that it works at the JW in Phuket, and the AC in Tokyo Ginza. Those are the only 2 times I've ever tried though.
I find it annoying that you need to open the app and enable bluetooth each time to get it to work. Hyatt has it where you can just add the mobile key to Apple wallet, which allows you to use it with just a double-click of the power button - no need to open any apps. I'd definitely use the mobile key more if Marriott added Apple wallet integration.