Adding issues to an issue board via the Add issues button doesn't work

Summary

Steps in https://docs.gitlab.com/ee/user/project/issue_board.html#add-issues-to-a-list don't work, as the list dropdown doesn't appear and the Add 1 issue button does nothing.

Steps to reproduce

Encountered in Chromium on Linux.

This happened in two specific issue boards I created:

• In gitlab for updating screenshots in the docs: https://gitlab.com/gitlab-org/gitlab/-/boards/2134049
• In GDK, a new board in http://127.0.0.1:3001/root/gitlab-shell/-/boards, with lists: ~Backend, Assignee: root, ~Frontend, ~Staging, ~Production.

1. Open a specific project issue board.
2. Click Add issues
3. Select an issue.
4. Click the Add 1 issue button

Example Project

What is the current bug behavior?

The list dropdown doesn't appear and the Add 1 issue button does nothing.

The browser console returns error:

TypeError: e.selected.label is undefined
    ListsDropdown lists_dropdown.vue:1

What is the expected correct behavior?

There's a list dropdown and the button adds issues to the board.

Relevant logs and/or screenshots

Output of checks

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`)

$ sudo -u git -H bundle exec rake gitlab:env:info RAILS_ENV=production

[sudo] password for marcin: 
sudo: unable to execute /home/marcin/.rvm/gems/ruby-2.7.2/bin/bundle: Permission denied

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

Console spits out this error...

Following the stack trace right before the error, there is no this.selected.label

It looks this this is a milestone issue list but the function is expecting an attribute from a label list, so it breaks.