Cannot apply != filter on merge requests

Summary

When doing a search on a Merge Request page, such as your very own here: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/?sort=created_date&state=opened&not%5Blabel_name%5D%5B%5D=Technical%20Writing&first_page_size=20

If you do Label != Technical Writing (Or any label), you get an error and the filter is not applied.

This is happening on our Merge Requests as well as pretty much any other one I've tested, including Tortoise Git here: https://gitlab.com/tortoisegit/tortoisegit/-/merge_requests/?sort=created_date&state=opened&not%5Blabel_name%5D%5B%5D=Performance&first_page_size=20

Steps to reproduce

  1. Navigate to a Merge Request page such as here: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/?sort=created_date&state=opened&not%5Blabel_name%5D%5B%5D=Technical%20Writing&first_page_size=20
  2. In the filter box choose Label
  3. For operator choose !=
  4. For the actual label, choose anything.

Example Project

Again, no need for sample your very own repo does it here: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/?sort=created_date&state=opened&not%5Blabel_name%5D%5B%5D=Technical%20Writing&first_page_size=20

What is the current bug behavior?

If you do Label != Technical Writing (Or any label), you get an error and the filter is not applied.

What is the expected correct behavior?

No error and filter is applied

Relevant logs and/or screenshots

Not sure you need these, since clicking on the link pasted in here at least 3 times instantly creates the error state.

Output of checks

This bug happens on GitLab.com

Results of GitLab environment info

Expand for output related to GitLab environment info 

(For installations with omnibus-gitlab package run and paste the output of:
`sudo gitlab-rake gitlab:env:info`)

(For installations from source run and paste the output of:
`sudo -u git -H bundle exec rake gitlab:env:info RAILS_ENV=production`)

Results of GitLab application Check

Expand for output related to the GitLab application check 
(For installations with omnibus-gitlab package run and paste the output of:
sudo gitlab-rake gitlab:check SANITIZE=true)


(For installations from source run and paste the output of:
sudo -u git -H bundle exec rake gitlab:check RAILS_ENV=production SANITIZE=true)


(we will only investigate if the tests are passing)

Possible fixes