Group rows which are not present in the view are not visualized when the window is maximized.
A possible solution here is to set the GroupRenderMode property to Flat.
Hi,
The fix for this problem is GridView's GroupRenderMode.Flat property. This is the newer virtualization mechanism of RadGridView and has a lot of improvements and fixes compared to the older Nested(which is the default one). If by some reason you need to stick to the Nested mode, you can open a new support ticket and we will try to find a solution that will work for the Flat mode as well.
Regards,
Yoan
Progress Telerik