They seem to do this quite frequently....happened to me as well.
I complained at reception and got my originally booked SNA panoramic room.

This hotel's "bait and switch" reservation confirmations nonsense should be investigated by Marriott corporate, as this is something I have not seen done at any other Bonvoy hotel.

Any recent stay?
A five night award stay costs 184,000 points, vs. 241,600 points at Sheraton Grand. The latter has much smaller rooms, but everything else is better than this hotel. Does it have a lounge?

Stayed last month (March) for six nights for an award redemption at the Park Tower Knightsbridge. No lounge, but there is a breakfast room for elites who choose it as a welcome amenity. Sit-down service, full hot buffet and can also order French Toast off a menu (other menu items, like omelettes, are for a charge). Not an extensive buffet but enough of a variety and quality to make it very pleasant.

There is also an in-room coffee and tea service, as you'd expect in a hotel of this type.

Location is excellent: right by the Knightsbridge tube station. And within a few minutes walking distance, if you're into picking up good snacks or to-go meals, is a Waitrose & Partners neighborhood grocery store, as well as the Harrods gourmet grocery. Right next door is the top-floor cafe at Harvey Nichols with extremely reasonable, and good, sit-down meals.

We were upgraded to a Panoramic Room which, while apparently the same size as most rooms, had a nice view of the adjacent park and appears generous in size for London. It was very comfortable. If you do reserve and don't smoke, be aware there are smoking floors at this hotel. But we requested a non-smoking floor and never smelled any cigarette smoke on our floor or in the lobby and common areas.

Thank you very much for the useful information about this property.
I have finally decided on staying at Marriott County Hall because its rate has fallen to the equivalent rates in points between Park Tower Knightbridge and Sheraton Grand. Marriott County Hall has the best of these two properties and I am happy with the choice. Of course, I cancelled the booking at Marriott Maida Vale, too, although its rate was only about $200 per night.