I'm having the exact same issue right now with LHR. Booking a few nights stay later this month and only one (yes, one) airport property is showing up on the website as available. If I search via the mobile app every single property shows up. First time I've actually had this happen to me.

The Courtyard Heathrow shows up in both searches. The Ren, Marriott, Sheraton, Sheraton Skyline, etc. all appear if I do the search for the exact same dates on the mobile app.

Utterly broken dumpster fire of a website is what I keep saying.

-RM