I didn't realize this was a thing with Marriott... "pre-confirming" your upgrade (in the app to boot - was there an email as well?) and then you don't actually get it.
I took a break from this Marriott forum for a couple of years... looks like I'll need a little more time getting over being Bonvoyed since I delayed that feeling. With Starwood, when I got my upgrade confirmation email, I actually got that room type (or even better) when I check-in (There might have been one or two fails in there out of over a thousand upgrades).