When the tag \sl (line spacing) has value 0, according to the specification it should be calculated according to the highest character in the line, which corresponds to the LineSpacingType single. Instead, in RTB this text disappears.

Workaround: 
private void RadRichTextBox_DocumentChanged(object sender, EventArgs e)
{
    foreach (var paragraph in this.radRichTextBox.Document.EnumerateChildrenOfType<Paragraph>())
    {
        if (paragraph.LineSpacing == 0)
        {
            paragraph.LineSpacing = 1;
        }
    }
}

Fix available in LIB Version 2017.3.1211.