How to reproduce: also check the attached video
this.radMaskedEditBox1.MaskType = MaskType.Numeric;

Workaround:
private void RadMaskedEditBox1_TextChanged(object sender, EventArgs e)
{
    if (this.radMaskedEditBox1.Text == "-0")
    {
        if (this.radMaskedEditBox1.MaskedEditBoxElement.TextBoxItem.Text.Length == 2)
        {
            //Workaround
            this.radMaskedEditBox1.MaskedEditBoxElement.TextBoxItem.Text = this.radMaskedEditBox1.MaskedEditBoxElement.TextBoxItem.Text.Remove(1, 1);
        }
    }
}