SYMPTOM
When selecting a Note box on a mapping diagram (or on a process diagram), the Expression Editor does not show the Note's content and therefore it is impossible to edit the Note from the Expression Editor.
SOLUTION
Notes created before 17.2.1 were not editable from the Expression Editor.
The mappings (and processes) imported from a pre-17.2.1 version may contain this kind of non-editable Notes.
There are two workarounds:
1. Click the contents of the selected Note and edit directly inside the Note's box
2. Click the contents of the selected Note and copy/paste it to a new Note, and then delete that old Note.