Existing events in my calendar do not seem to be considered for availability

Describe the bug
I am able to book an appointment during a conflicting event in my calendar. I would expect this not to be possible.
None of my existing events seem to be considered for my availability.

To Reproduce
Steps to reproduce the behavior:

(only tested on https://appointment.day/ as deployed at the time of this issue creation)

My calendar is a caldav shared from a private nextcloud instance.

1. connect app to an existing caldav calendar with some events through the initial setup workflow.
2. configure appointments app with availability from 9-20 Mon-Fri.
3. in appointments app, try to book an appointment during an existing event. No existing event seems to be considered for availability.
4. Notice the tentative event notification being received by the owner of the calendar.

Expected behavior

Time slots that conflict with events in the calendar should not be available to book.

Actual behavior

I am able to book appointment during any of the existing events in my calendar.

System (please complete the following information):

• OS: debian sid
• Browser Version firefox 133.0.3 (64bits)

Additional context
Add any other context about the problem here.