A while back I had a reservation with a hotel on Booking and I received a phish attempt that came directly via the Booking site domain email and also DMs but "sent" by the hotel. When I looked into it at the time, it seemed less like an issue of hotels specifically having their accounts infiltrated and more like some kind of message/email endpoint on Booking's end was being abused in a similar manner.

I'm not sure this is the same type of issue but found this interesting, especially since apparently it's been reported to MS and no action has been taken.