Workaround: 
public Form1()
{
    InitializeComponent();
    this.radMaskedEditBox1.MaskedEditBoxElement.TextBoxItem.KeyPress += TextBoxItem_KeyPress;
}

private void TextBoxItem_KeyPress(object sender, KeyPressEventArgs e)
{
    if (e.KeyChar == '-')
    {
        this.radMaskedEditBox1.MaskedEditBoxElement.TextBoxItem.SelectionStart = this.radMaskedEditBox1.MaskedEditBoxElement.TextBoxItem.Text.Length;
    }
}