Postback redirect through add_action is not triggered

Let’s say your site’s address is example.com. When you want to redirect to example.com/page/subpage you should use site_url('/page/subpage').

I assume the WordPress address is http://127.0.0.1:8000. You have in your code
site_url( 'http://127.0.0.1:8000/?page_id=5' ) and it will be converted to this address: http://127.0.0.1:8000http://127.0.0.1:8000/?page_id=5.

Try using this redirection:

wp_safe_redirect( site_url( '/?page_id=5' ) );

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Second issue, if you’ll take a look at code reference, you will find there:

wp_safe_redirect() does not exit automatically and should almost always be followed by exit.