Hi guys, doesn't seem like telerik was much help on this at all. I encountered the same issue in my project where I was setting the ColumnGroupDescriptor of a RadGrid GroupDescriptor through my XAML. At some point the screen would rebind the RadGrid to null and then back to a valid list. At this point my grouping was completely lost.
I hate programatically setting binding but in this instance, I passed the RadGrid to my ViewModel as a method parameter from a trigger event and in the corresponding method linked to that trigger, I set the itemsource and a new GroupDescriptor. This worked and allowed the grouping to reappear each time the Grid was set to null and then back to a list.I had to remove the GroupDescriptor definition in the XAML as well.
Sample of C# code to set Group Descriptor Programatically:
GroupDescriptor descriptor = new GroupDescriptor();
descriptor.Member = "NAME_OF_MY_COLUMN";
descriptor.SortDirection = ListSortDirection.Ascending;