This is yet another taxonomy issue caused by what users can and cannot see when inside a scope.

Steps to reproduce:
1) log in as admin
2) create john as an admin with no location/organization
3) assign john as an owner of some-host.example.com (host is in Default Organization/Default Location)
4) create beth with manager role in Default Organization/Default Location
5) log out, log in as beth
6) edit some-host.example.com, go to 'Additional Information' tab
7) notice john is not set as owner (dropdown has a blank value set) and is not available in dropdown selection
8) Hit the submit when owner dropdown has empty value selected, revisit 'Additional Information' again
9) beth is now owner of some-host