The work around for this issue is to set the SpacingAfter property to 0 and LineSpacing property to 1:
RadDocument doc = provider.Import(RTF);
foreach (Paragraph p in doc.EnumerateChildrenOfType<Paragraph>())
{
p.LineSpacing = 1;
p.SpacingAfter = 0;
}
this.radRichTextBox1.Document = doc;
The same thing is happening on the bulet list when load content from html (<li> element).