To reproduce:
Use the following code and then select some text:
private void radButton1_Click(object sender, EventArgs e)
{
    TxtFormatProvider txtProvider = new TxtFormatProvider();
    RichTextEditor.Document = txtProvider.Import(sampleText);

    RichTextEditor.Document.LineSpacingType = LineSpacingType.Exact;
    RichTextEditor.Document.LineSpacing =10;

    DocumentPosition startPosition = RichTextEditor.Document.CaretPosition;
    DocumentPosition endPosition = new DocumentPosition(startPosition);
    startPosition.MoveToStartOfDocumentElement(RichTextEditor.Document);
    endPosition.MoveToEndOfDocumentElement(RichTextEditor.Document);

    RichTextEditor.Document.Selection.Clear();
    RichTextEditor.Document.Selection.AddSelectionStart(startPosition);
    RichTextEditor.Document.Selection.AddSelectionEnd(endPosition);

    RichTextEditor.RichTextBoxElement.ChangeFontFamily(new Telerik.WinControls.RichTextEditor.UI.FontFamily("Segoe UI"));
    RichTextEditor.RichTextBoxElement.ChangeFontSize(Unit.PointToDip(10));
    RichTextEditor.Document.Selection.Clear();
}

Workaround:
    RichTextEditor.Document.LineSpacingType = LineSpacingType.Auto;
    RichTextEditor.Document.LineSpacing =.5;