I attempted to reproduce this issue on 6.1 GA1, but could not.
- Assign a test user to an organization with Organization Administrator role.
- Sign in as test user.
- Go to Control Panel -> Users and Organizations
- Select Actions -> Assign Users on the organization
- Un-select the test user (self) and save.
The test user is successfully removed from organization and the user no longer retains administrative privileges over the organization. Please ensure you have a fresh environment with no other issues that will interfere with this issue. If you still experience the issue please provide me with detailed steps so that I may verify the bug. Thanks!