Hello, Karsten,    

RadGanttView internally uses binary search for traversing the parent and child IDs and it replies on the fact that the IDs are sorted. You may obtain incorrect hierarchical structure even with integer IDs if the values of the parent and child records are not ordered.

We will try to introduce a fix as soon as possible. However, I can't give you an exact time frame when the issue will be addressed. Make sure that you follow this item. Thus, you will get notified once the status changes.

Should you have further questions please let me know.

Regards,
Dess | Tech Support Engineer, Sr.
Progress Telerik

Hello, Karsten,     

Following the provided information, I was able to replicate incorrect data-binding behavior when using Id/ParentId that are not integers, e.g. Guid. Some of the task items are missing or the hierarchy is not built properly. The behavior is absolutely random. I have attached a sample project demonstrating the issue. The below screenshot illustrates the expected behavior. If you run the application several times, you are going to obtain different result. If you try to hide some of the columns on the left grid, the behavior also is different.

I have logged it in our feedback portal by making this thread public. You can track its progress, subscribe for status changes and add your comments on the following link - feedback item.

I have also updated your Telerik points.

Currently, the possible solution that I can suggest is to use integer values instead.

Regards,
Dess | Tech Support Engineer, Sr.
Progress Telerik