This user https://hello.atlassian.net/people/5af3c18b7b44642d39d3ade7 is an add-on 'System' user.

It is appearing in the restrictions menu along with what appear to be all add-on users.

I feel that 'System' users should not appear in user pickers for restrictions, @ mentions, share menus etc. as otherwise it causes confusion for staff in using these Confluence features, especially if an add-on is named something human-like.