We are seeing an issue where setting the GroupRenderMode to "Flat" causes the nested row hierarchy details (inside a HierarchyChildTemplate) to disappearing after toggling the details. This problem goes away after setting the GroupRenderMode to "Nested".
This is strange behavior and we are not aware of what could lead to this behavior. I have double check this on my side and everything works as expected. I may miss something from your implementation set up. You can find the project which I used for testing purposes attached to this reply. Can you take a look at this project and let me know what I need to modify to reproduce this behavior.
I am looking forward to your reply.
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