Epic (re)assignment from the swimlanes issue sidebar is unreliable and usually triggers an error

Summary

(Re)Assigning the epic of an issue from the sidebar/drawer when swimlanes applied doesn't seem to reliably work. I am encountering an error often.

swimlanessidebarepic

Steps to reproduce

Open an issue board
Apply swimlanes
Click on an issue to open the sidebar
Click into the epic input within the sidebar

Example Project

https://gitlab.com/gitlab-org/gitlab/-/boards/1548297?label_name[]=devops%3A%3Aplan&group_by=epic

What is the current bug behavior?

Clicking into the epic input in the sidebar opens the dropdown but triggers an error banner.

What is the expected correct behavior?

The user is able to see a list of epics within the dropdown, then click an epic to assign it to the issue. No error banner should ideally appear. If it does- we should improve it with #282141 (comment 446761838)

Relevant logs and/or screenshots

See video above

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`)

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)