Preside's checks to see if a form has unsaved changes before navigating away from a page do not see changes made within a CKEditor rich text field.
This is probably because the underlying textarea field is only updated when the form is submitted.
2 ways to solve this:
1. Update the underlying text area when the editor loses focus
2. Hook in to the CKEditor API which should be expose whether the editor instance has changes.
If (2) is possible, this would be the preferred route.