Bug: Able to select non members user from private project issue boards

Summary

Ability to select and assign non-members user to private project boards

Steps to reproduce

Go to Boards in any private project and attempt to assign it to an external user, a selection of any external user will appear however if you try to assign it to an external user it won't happen.

If you try the same behavior from within a private project issue itself it is not possible to do.

Example Project

https://gitlab.zendesk.com/agent/tickets/195047 (internal)

What is the current bug behavior?

You can select anyone on private project issue boards when clicking an issue in the project boards and fill the dropdown menu of assignee

What is the expected correct behavior?

Only members of the project should be able to be available to pick up

Output of checks

This bug happens on GitLab.com

Results of GitLab environment info

GitLab 13.8