Problem when changing assignee on 2nd issue

Summary

Steps to reproduce

  1. using gitlab free
  2. there are more than 2 issues in issue list
  3. issues is assigned to user01
  4. change issue-1's assignee from user01 to user02 (it will be successfully)
  5. change issue-2's assignee from user01 to user02 (it will be error)

Example Project

What is the current bug behavior?

at step 5, gitlab will showed that 2 users has been assigned to issue (multi assign), but Free version doesn't has this feature. Then the assignee still is user01 instead of being changed to user02

What is the expected correct behavior?

issue-2's assignee should be changed to user02

Relevant logs and/or screenshots

image

Output of checks

This bug happens on GitLab.com