To reproduce:
Import a rtf document and insert the imported content as follows:

RtfFormatProvider rtfFormatProvider = new RtfFormatProvider();
RadDocument doc = rtfFormatProvider.Import(File.ReadAllText("WordpadRTF.rtf"));
doc.Selection.SelectAll();

rtf2.Document.InsertFragment(doc.Selection.CopySelectedDocumentElements(), rtf2.Document.CaretPosition);

You will notice that there are no empty lines, edit the text and the lines will appear again.

Resolution: 
This issue is addressed in the new version of the control - RadRichTextEditor. Please use the new control instead the RadRichTextBox.