How to reproduce: 
public Form1()
{
    InitializeComponent();

    this.radMaskedEditBox1.Culture = new CultureInfo("ar-EG");
    this.radMaskedEditBox1.Mask = "c";
    this.radMaskedEditBox1.MaskType = Telerik.WinControls.UI.MaskType.Numeric;
}

Workaround: handle the KeyDown event
 private void radMaskedEditBox1_KeyDown(object sender, KeyEventArgs e)
 {
     if (this.radMaskedEditBox1.SelectionLength == this.radMaskedEditBox1.Text.Length)
     {
         this.radMaskedEditBox1.SelectionLength = 0;
     }
 }