Activities that have a trailing whitespae character in their name cause a JavaScript error and the dropdown will not populate and the column will not hide. It also will not appear when revisiting the page if a column is collapsed. Whitespace appears to be stripped from activity names in recent versions of Moodle so this likely applies to older activities where the name was pasted with a trailing space.

Steps

1) Add a gradeable activity to your course
2) Force whitespace on to the end of the activity name by appending " "
3) Visit grader report
4) Collapse an item
5) Click the collapsed columns menu

Expected

The column is hidden from the report. The Collapsed columns dropdown is shown and is populated with the collapsed column.

Actual

The column remains visible in the report. The Collapsed columns dropdown is shown with 3 column place holders

Console output

first.js:4084  
Uncaught (in promise) Object
errorcode: "invalidresponse"
link: "https://.../grade/report/grader/index.php?id=3729"
message: "Invalid response value detected"
moreinfourl: "http://docs.moodle.org/403/en/error/debug/invalidresponse"
[[Prototype]]: Object