We are aware of this issue. The edit page properties page is just like a normal SharePoint form. However, it is not using the RichTextField template for rendering the editor. The same problem also occurs when, for example, you add/edit an item in the Reusable Content list. Since this is not the normal list editor, we cannot override it with RadEditor even if the editor features are active from the site features page. For now, this is an unsupported scenario for RadEditor.
the Telerik team