Hi Uwe,
Greetings from Syncfusion Support.
We checked the reported problem “Timeline and Timeline Grouping broken” and we are able to replicate the issue. So we considered this as a defect and logged the below defect report. The fix for this issue will be available in our upcoming patch release which is scheduled on October 14, 2020. We appreciate your valuable patience until then.
Regards,
Vengatesh