Hello Maurice Lohse,
Thank you for your post.
I have done some testing from my end too. I thought the issue could be related to propagation, issue but it's not.
I was able to reproduce the issue from my end. I waited for 30 minutes after I made changes to the booking via Graph PATCH but the updated event info is not visible with 'https://graph.microsoft.com/v1.0/me/calendar/getSchedule' call. It shows old Subject.
The issue must be related to a bug therefore, I would recommend you create a support ticket by tagging Microsoft Graph.
Here is the link to open a ticket;- https://developer.microsoft.com/en-us/graph/support.
Let me know, if that answers your question.