This is my guess as well. They have been limiting new signups for the paid service for a long time, which must mean they are overloaded; and then it makes a lot of sense to just degrade the quality of GPT-4 so they can serve all paying users. I just wish there was a way to know the “quality level” the service is operating at.
Better yet, demand loudly to get a refund. When they say there is nothing to refund, insist that you have an email confirming a booking.