Cannot rescale forum's maximum grade after one student is graded

One of our teachers fell on this one today. When she tried to rescale her forum's maximum grade, Moodle didn't even show the rescale menu... I was able to reproduce this systematically on Moodle 3.1.2, 3.1.3 and 3.2 DEV.

Follow theses steps :

1. Create a forum with ratings.
2. As a student, write a post.
3. As a teacher, grade the post.
4. Go back to the forum's setting and try to rescale the maximum grade.
   Result: There will be no rescale menu (yes or no). This makes it impossible to rescale the activity because it will give an error.