My first guess was switching to sameSite: 'none' on the cookie since this is a post request, but they're both on the same domain (herokuapp.com), because it's probably not being passed, but that doesn't seem to make a difference.
#Session not passing in prod
wooden elbow
wooden elbow
Ok I just uploaded all my code to Railway and it does it on that too, so it doesn't seem like it's a specific to Heroku issue