There are 2 locations where the tester can be assigned and maybe you're looking at the 'other' one.
In the screenshot you shared that's the owner of the test case. To see the tester assigned you can either go to the test script steps (each step can be assigned to a tester), or you could go into the test player (click on the test cycle) and view the script there. If you wanted to amend testers in bulk you would go into the test player (click on the test cycle) > click on the test cycle ID in the breadcrumb trail > go to test cases > view/edit testers
