Bulk editing group issues will unassign any assignees on the issue

Summary

When bulk editing issues under a group (even if you are editing one issue), if any changes are applied via this process, GitLab unassigns any assigness from the issue.

Steps to reproduce

  1. Create a new premium/ultimate group on SaaS
  2. Create a new milestone under the group
  3. Create a new project under the group
  4. Create a new issue under the project. Assign yourself to the issue.
  5. Head to the issues page under the group.
  6. Follow the documented process to edit the issue we created and add it the milestone.
  7. Observe that the assignee has been removed from the issue.

Note: This also happens when you edit the issue with a label, or add it to an epic.

Example Project

https://gitlab.com/groups/gitlab-silver/kenneth-silver/-/issues

What is the current bug behavior?

Assignees are unassigned from bulk edited issues from the group page.

What is the expected correct behavior?

Assignees are not unassigned.

Relevant logs and/or screenshots

Screenshot image

Red rectangles indicate changes made as part of the same bulk edit. Assignee should not have changed

Output of checks

This bug happens on GitLab.com - GitLab Enterprise Edition 15.9.0-pre c24f8825

This was not reproducible on 15.7.6 and 15.8.2.

Possible fixes

Possibly due to Replace user_select dropdown in Issues list bul... (!108509 - merged) which was merged into production on 1st Feburary?