To reproduce: please refer to the attached gif file demonstrating how to replicate the problem with the Demo application. Scroll with the mouse wheel and you will notice the the row index disappears until you click a cell. Using the thumb will behave as expected. 

Workaround:
this.radSpreadsheet1.SpreadsheetElement.VerticalScrollBar.ValueChanged+=VerticalScrollBar_ValueChanged;
        private void VerticalScrollBar_ValueChanged(object sender, EventArgs e)
        {
            this.radSpreadsheet1.SpreadsheetElement.InvalidateMeasure(true);
        }