To reproduce:
- Set the mask to numeric.
- Change the value and check the Modified property.

- Workaround:
string oldValue = string.Empty;

void radMaskedEditBox1_LostFocus(object sender, EventArgs e)
{
    if (oldValue != radMaskedEditBox1.Text)
    {
        radMaskedEditBox1.Modified = true;
    }
    else
    {
        radMaskedEditBox1.Modified = false;
    }

    oldValue = radMaskedEditBox1.Text;
}