Appointments App: Cross-checking multiple calendars for free/occupied does'nt work reliably

Dear nextcloud community,

after more than two hours of unsuccessful searching, I am now turning to you with my question.
I hope you can help me. It’s about the nextcloud appointments app for appointment booking.

Current state:
At the moment my nextcloud appointments app checks availabilies against 4 calendars.
Free slots are then shown, but for example an appointment from 9-10h can be booked, although an appointment from 9.00-9.15h is already entered in 1 of the 4 calendars.
Additionally, sometimes appointments that are only released with the status “busy” are not “respected” and can be overbooked.
I can’t see a consistent pattern.

The default length of a bookable appointment is 1h
All calendars are manually set to Berlin timezone.
For all calendars the user on which the appointments app is started has write permissions.

Desired state:
Only time slots of at least one hour that are actually free in all four overlaying calendars should be displayed and be bookable.

What is the best way to achieve this?
Thanks in advance,

PS: NextCloud, Calendar and Appointments App are on their latest stable versions.

I am facing the same issue - the selection process for the external user does not check the conflicts in other calenders assigend in the appointment app. It just shows all existing time slots from the planning. Even if you are calling the planning tool from external it shows all as available. As a consequence there are overlapping appointments booked.

Does anybody have solved this?