In my case, "+" sign get stripped out.

To reproduce :

1. Have a course with an assignment and a couple of student
2. Log in with a teacher user
3. Go in grader report (Course administration > Grades)
4. In Grade administration ► My report preferences ► Grader report, make sure you have the following options enabled:
   1. Quick grading = Yes
   2. Show quick feedback = Yes
   3. Enable AJAX = yes
5. Turn Editing On (doesn't matter actually but made test easier)
6. Add "+++" in the quick feedback field for a student
7. Change page (go to the outcome report)
8. Return to grader, quick feedback for the previous student will be empty

I've test a couple of characters and I found that plus sign and amperstand (&) signs get stripped out. Others, like minus, div, dollar sign, etc. are ok.