The issue is not reproducible with GroupRenderMode.Flat, which is the newer and recommended UI virtualization engine for RadGridView. Please open a support ticket if you have any difficulties with migrating to it.
As a solution, I can suggest you configuring RadGridView with GroupRenderMode="Flat". The Flat mode was introduced with Q2 2013 as a completely new logic for group rendering. The result was faster scrolling and very good performance, but it fixed a lot of other issues in group mode reproducible with the old Nested mode.
This is high-priority for us, as it makes the grouping feature on the grid unusable for our application and we were counting on using it.