I just answered your support ticket on the subject. For your convenience I have pasted below my answer:
Thank you for reporting this bug.
I was able to reproduce it and logged it for fixing in our bug tracking system. Here you can find the PITS Issue: Public URL. I also updated your Telerik points for the bug report.
You can try to workaround the issue by loading the content in RadEditor in the OnClientLoad client event using the editor.set_html(content) method.
the Telerik team
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