Steps to reproduce:

1. Set up Solr Search Engine, see documentation.
2. Create a user but don't assign him anywhere yet.
3. Create any custom service which uses:

       /** @var \eZ\Publish\API\Repository\UserService */
       private $userService;
   

4. Play around with the following code:

           $user = $this->userService->loadUser(59);
   
           $adminUserGroup = $this->userService->loadUserGroup(12);
           $this->userService->assignUserToUserGroup($user, $adminUserGroup);
   
           //$this->userService->unAssignUserFromUserGroup($user, $adminUserGroup);
   

5. Observe the Users tab in the Back Office.

Result:
You cannot see the changes you have done via the API.

Expected result:
You should see changes being reflected in the Content Tree.

Note:
Reindexing the content fixes this issue.
Affected methods:
assignUserToUserGroup, unAssignUserFromUserGroup