RadGridView's GroupRenderMode.Flat is the newer and improved option for vertical UI virtualization. It features much better performance in certain scenarios, allows the usage of the newest features of RadGridView, and also aims for full parity with the other features that are supported by both RenderModes. To put it briefly, GroupRenderMode.Flat is the better option in the vast majority of cases and we are currently focusing our development effort to it. Thus I strongly recommend that you migrate to it. If you need any help with the migration process, do not hesitate to use our support ticketing system or forums.
Please explain why this "not supported" and won't be fixed
I also have a vested interest in seeing this bug resolved.
Why is the status "Won't fix?"
This functionality is not supported for RadGridView.GroupRenderMode="Nested". We recommend using RadGridView.GroupRenderMode="Flat".