Subrequests in an KernelEvents::TERMINATE event subscriber (f.e. cron tasks doing subrequests) try to restart a session, resulting in runtime exceptions

FWIW, I'm having similar issues because I'm implementing support for sending content updates via websockets on the tail of requests (looking at which cache tags were invalidated). The particular scenario I'm currently facing is that I've serialised the render array for a view when the user initially rendered the page. Now another request has modified the content and I'm seeking to re-render the view, but it has an exposed form and Formbuilder is seeking to check whether the session has a batch form state (line 253 of FormBuilder.php).

Fun!