When using the feedback activity, the preview icon is displayed for everyone in the course, including students. This is especially worrisome in the app, where the resulting "Preview" button has the same size as the button to actually submit your feedback. After clicking on "Preview", users then believe that they are actually filling out the feedback form, but finishing this process will not actually submit the feedback.
The code is in mod/feedback/view.php with the assignment of the variable $preview.
I suggest adding a check so that $preview is an empty string when the user does not have the permission to edit the activity.