I am not sure about changes to event.response not sticking. I was able to set event.response.resp_check = true; statement and when I return the event.response resp_check was there intact in the response printed in console.
The problem I have is about the platform.session.user and the patch event. For me platform.session.user returns null and not working because I have setup a check inside the if loop if(!!platform.session.user) which is not going through whereas the above resp_check is set outside the loop and this is being set successfully.
So, event there is a bug with platform.session.user being null while the time of registering in the POST process script.