To reproduce:
- add a RadMaskedEditBox with MaskType = MaskType.Standard
- input some numbers to fill all the expected digits
- select the content in the RadMaskedEditBox and press Enter key. As a result the last number is removed

Workaround:
private void radMaskedEditBox1_KeyPress(object sender, KeyPressEventArgs e)
        {
            e.Handled = e.KeyChar == (char)Keys.Return;
        }