Steps to reproduce:
- Login with Admin
- Create 2 global users: U1 & U2
- Go to Administration -> Wikis and install the Wiki Template
- Create a new subwiki and in step 2 set U1 as the owner and U2 as a member
- Hover over Wiki and click "Administer Wiki"
- Click Wiki
- Change the subwiki's owner to U2
- Click Save
- Click Groups
- Click XWikiAdminGroup
You'll see both the previous and the current users (U1 & U2) listed.