Hello, Mohsin,

Thank you for reporting this. You can use the following workaround and explicitly set the editor's culture to be the same as the GridViewMaskBoxColumn:

private void RadGridView1_CellEditorInitialized(object sender, GridViewCellEventArgs e)
{
    RadMaskedEditBoxEditor editor = e.ActiveEditor as RadMaskedEditBoxEditor;
    if (editor != null)
    {
        editor.MaskTextBox.Culture = CultureInfo.CreateSpecificCulture("en-AE");
    }
}

I hope this helps. If you have other questions, please let me know. 

Regards,
Nadya | Tech Support Engineer
Progress Telerik