Thank you for writing.
RadGridView uses UI virtualization
for its cells, which means that there are only certain amount of cells visible at all times and the data in these cells is being changed when scrolling, hence it is necessary to reset each visual settings you apply. More information and examples you can find here: http://www.telerik.com/help/winforms/gridview-cells-formatting-cells.html
In your case, you should add the following code at the end of your CellFormatting event handler:
I hope that you find this information useful. Should you have any other questions, do not hesitate to contact us.
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