should webhooks deployed on heroku work the same?

Question

@app.route('/hswebhook', methods=['POST'])
def webhook():
    if request.method == 'POST':
        test = request.json
        print(test)
        return 'success'

Using postman, and testing this locally this works as expected. However when I deploy on heroku and when I use API's who's callback url is set to this, it no longer works. The console log shows this:

2022-09-14T18:43:13.553790+00:00 heroku[router]: at=info method=POST path="/hswebhook" host=runn1.herokuapp.com request_id=dfd91ea5-5c17-41ff-b209-961179a1f7dc fwd="34.198.117.22" dyno=web.1 connect=0ms service=1ms status=400 bytes=337 protocol=https
2022-09-14T18:43:13.556133+00:00 app[web.1]: 10.1.20.108 - - [14/Sep/2022:18:43:13 +0000] "POST /hswebhook HTTP/1.1" 400 167 "-" "HelloSign API"