Appointment is deleted when clicking "hide" in notification

I am using an Exchange calendar with the em client “9.2.2093 (6d2fe6b)”. 15 minutes before an appointment I see a reminder with two German options: “Zurückstellen” (postpone) and “Ausblenden” (hide). As I simply want to close the reminder, I click the button to hide. But that does not hide the reminder, but completely deletes the appointment from my calendar.

Is this a feature, a setting or a bug?

(It is really confusing, because you only want to close the reminder, but afterwards the whole appointment incl. all contact information is completely lost. That is very weird.)

Thanks for some advice, how to avoid this.