To reproduce:

RadDocument document = new RadDocument();
Section section = new Section();
Paragraph p = new Paragraph();
Span span = new Span();
span.Text ="test" + Environment.NewLine + "test";
ReadOnlyRangeStart rangeStart = new ReadOnlyRangeStart();
ReadOnlyRangeEnd rangeEnd = new ReadOnlyRangeEnd();
rangeEnd.PairWithStart(rangeStart);
p.Inlines.Add(rangeStart);
p.Inlines.Add(span);
p.Inlines.Add(rangeEnd);
section.Blocks.Add(p);
document.Sections.Add(section);
this.radRichTextEditor1.Document = document;


Workaround: add an empty span as last inline element in the paragraph:

RadDocument document = new RadDocument();
Section section = new Section();
Paragraph p = new Paragraph();
Span span = new Span();
span.Text ="test" + Environment.NewLine + "test";
ReadOnlyRangeStart rangeStart = new ReadOnlyRangeStart();
ReadOnlyRangeEnd rangeEnd = new ReadOnlyRangeEnd();
rangeEnd.PairWithStart(rangeStart);
p.Inlines.Add(rangeStart);
p.Inlines.Add(span);
p.Inlines.Add(rangeEnd);
///////////////////////////////////////
Span emptySpan = new Span();
emptySpan.Text = " ";
p.Inlines.Add(emptySpan);
///////////////////////////////////////
section.Blocks.Add(p);
document.Sections.Add(section);
this.radRichTextEditor1.Document = document;