In this thread many reasons for the content missing on post back behavior has been discussed and I can only assume what exactly is the behavior encountered on your end.
The most commonly encountered issue is due to the usage of button that creates the post back. This matter is explained in this forum thread
and possible solutions are provided. Note that this behavior is not a bug, but a browser behavior relate to the button element which does not raise the submit event and teh RadEditor cannot save its content properly to preserve it through post backs.
Also, a common mistake that lead to the same behavior is when the content is being overridden in the code behind.
If you still have issue, please send us a proper support ticked with more details provided, so that we could investigate the exact reason for the encountered issue.
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