In Course Grade Override, the instructor is not able to introduce more than 2 characters in the input.
This is not a problem when your institution is using one of the default grading schemas, as them are using two characters in maximum.
In my case, I'm using the next Grading Schema and is not possible to write "Excel·lent" in the Course Grade Override input:
The issue only affects to 11.x. It was solved on master in this commit: https://github.com/sakaiproject/sakai/commit/d21ee212db2b2e315b96f6ff0fffa3aeac8108a8 (CourseGradeOverridePanel.html)