We were having exactly the same problem where the TinyMCE editor was appearing in the description, but not in the page content. We're using Moodle 2.2.3 and it occured for pages that had been upgraded from Moodle 2.0 as well as new pages. It seemed to be only happening in some browsers (IE for us, and only some version/s - not sure which one/s).
The fix of changing the activity description to not be required (as suggested by Graham) works! I don't understand why this is, and too bad if you want it to be required, but for us this fix is fine.
Thanks Graham for suggesting this!