Self-book appointments should work for multi-patient appointment slots (eg 3 patients in 15 min slot)

The self-booking link does not work when it is sent out for a "muliti-patient" appointment slot. We run our flu clinics in "waves" so patients can come at 9am, 9:15am, 9:30am etc.

However, when sending invites via patchs for patients to self-book into these appointments, the patients receive an error message "no appointments available".

There are no issues with the single patient appointments slots.