Autocomplete endpoint: Can't assign user outside of project to an issue

In gitaly#1343 (closed), I was unable to assign a community user to an issue.

The autocomplete endpoint doesn't find this user:

https://gitlab.com/autocomplete/users.json?search=fabioperrella&active=true&project_id=2009901&current_user=true&author_id=64248

But if I drop project_id, it works:

https://gitlab.com/autocomplete/users.json?search=fabioperrella&active=true&current_user=true&author_id=64248