Pending confirmation appointments give wrong availability with addons

Have an appointment with 'check availability' set to last 1 day and add extra days with add-ons, no staff. After a user books 28.3. for 2 days (check availability), the 28th and 29th are not selectable on the calendar. That's ok. But if you select the 27.3. you can add 1 extra day, which is wrong.

Test it here for 27th. The 28th and 29th are booked by the same product. Even after confirming the appointment, it's the same.