In the feedback tabs of advanced settings, you can fill a WYSIWYG field named feedbacknotes.

If you upload images via the WYSIWYG, it's well dipslayed and still well displayed after saving the feedback.

But if you go back on the page later, the images are not displayed.

(I.E go to  questions tabs and go back on the feedback tab)

I found the bug :

The id (survey id $sdata->sid) used with file_prepare_draft_area function is not the same than the id (course module id $sdata->id) used with file_save_draft_area_files

https://github.com/PoetOS/moodle-mod_questionnaire/blob/MOODLE_39_STABLE/feedback.php#L75

https://github.com/PoetOS/moodle-mod_questionnaire/blob/MOODLE_39_STABLE/feedback.php#L106

I think the id to use should survey id.