Would you please try 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.
Basically, if column virtualization is on, then only the rows in the view area are available, the rest of them are realized on demand. On the other hand, if it is disabled, then all the visual containers are initially realized. You can check our online documentation explaining how the UI virtualization works.
Let me know the result.
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