The exact steps to reproduce are:
1. Use RadEditor in http://localdemos.telerik.com/aspnet-ajax-dev/editor/examples/overview/defaultcs.aspx
2. Add this HTML:
<p>start</p>
<p> </p>
<p>end</p>
3. Go to Design;
4. Select from the empty paragraph (<p> </p>) to the end of text (<p>end</p>);
5. Copy via Ctrl+C;
6. Try to paste at the end of the "end" paragraph.
A possible solution is to set the StripFormattingOptions property to NoneSupressCleanMessage. You can test it at https://demos.telerik.com/aspnet-ajax/editor/examples/cleaningwordformatting/defaultcs.aspx.