TTMSFNCRichEditor.InsertText expects single line text to insert.
If you want to add multi-line text, use TTMSFNCRichEditor.AddMultiLineText()
Example:
begin
memo1.Lines.Clear;
memo1.Lines.Add('First line');
memo1.Lines.Add('Second line');
TMSFNCRichEditor1.AddMultiLineText(memo1.Lines.Text);
end;
This works as expected.