I'm also running into a similar issue with the 3.5 .NET version (3.9 of the data grid).
The data grid is in a tab. If I switch tabs, then back to the data grid tab, and group or ungroup, the data grid freezes up and displays everything wrong. Only way to fix it is to cause it to Measure (by resizing the window) or by change tabs again.
In addition, if I make the window small (which resizes the datagrid to a smaller size), switch tabs, make the window large, then switch back to the data grid tab. The data grid will freeze up and stop resizing to fit the window any longer.