Future Task for recurring appointment ticket is not deleted

[wetzf]

When deleting a recurring appointment that is configured to create a ticket the corresponding future task is not deleted. So a ticket is created once after the deletion of the appointment.

Steps to reproduce:

• create a recurring appointment that creates tickets
• check "calendar_appointment" and "scheduler_future_task" tables to get ids
• delete appointment
• check "scheduler_future_task". the task is still there.

The issue seems to be caching. If I clear the OTOBO cache before deleting, the future task is deleted correctly.
When creating an appointment, the future_task_id is initially not set yet. So the appointment is cached without a future_task_id and those cached values will be used until the cache expires.