Interesting observation.
Booking.com just tried scamming me into paying 400€ more on a hotel just because I opened the page when signed in with my existing account. Opening the same page from a fresh session and *then* signing in gives me the discount again.
So make sure to search for your hotels without signing in first, I guess... or don't use booking.com - but let's be real, it's not like other sites don't do the exact same thing.
I hate this.