The reported issue does not seem to be reproducible when GroupRenderMode is set to "Flat", which is the newer and recommended UI virtualization mode for RadGridView. If the recommended workaround does not meet the requirements you can try to change a FontFamily to be different from "Arial" and "Calibri" and FontSize to be different from 14 or open a support ticket.
This item was originally instigated by the following ticket. http://www.telerik.com/account/support-tickets/view-ticket.aspx?threadid=824961 Please ensure that the fix for this item also fixes the issue reported in the above ticket.