Issues are unassigned when leaving project and being its group's owner

Summary

Issues that have been assigned to a user are unassigned when this user is leaving the project while still being its group's owner.

Steps to reproduce

1. Create a group
2. Create a project and add it to this group
3. Create an issue in this project and assign it to yourself
4. Add yourself as a member to the project (even though you are already one because you are the owner of its group)
5. Remove yourself as a project member again

What is the current bug behavior?

GitLab still displays you as a project member because you are its group's owner. So far, so good. But the issue that was assigned to you has been unassigned.

What is the expected correct behavior?

The issue should still be assigned to you IMO, because you are actually still a project member.

Output of checks

This bug happens on GitLab.com and in GitLab CE 9.5.1