The RadEditor WebPart is suitable to edit parts of the content on the page. When your click Edit Web Part button of the WebPart Menu and type content in the editor, it is recommended to click the OK or Apply buttons of the Telerik RadEditor for SharePoint WebPart toolbar. This is the correct and only way to save the content in the web part editor.
If you click Edit Page button from the top toolbar, while the web part editor is in edit mode, you will not also lose the RadEditor content but this will cause the reported SharePoint behavior. This is not something that we can control but a side effect of SharePoint.
That is why our recommendation is to firstly save the RadEditor's content from the WebPart toolbar and after that press the Edit Page button if you need to make additional changes to the page.
All the best,
the Telerik team
If you want to get updates on new releases, tips and tricks and sneak peeks at our product labs directly from the developers working on the RadControls for ASP.NET AJAX, subscribe to their blog feed