If you pass an ID token and an after logout redirect URI to /logout, it should work. You might have to do a redirect on the client side. For example, here’s how I’ve implemented it in Angular - where the /logout endpoint in my app returns an ID token and redirect URI.
You need to logout from the okta portal also for now your user is just getting logged out of your application and not from the okta portal you can check something for SLO in the OKTA