Amplitude events not shown for existing users in our database.We have been using Amplitude since the past 3 months and as User ID, we send the e-mail ID of our users.We decided to replace email in User ID with the actual ID of the user in our system {Ex: Sarah@gmail.co will be replaced by 120 (120 being Sarah's User ID in our systems)}. We are aware that this could potentially effect Retention metrics, and that's okay.Here is the issue I am facing. We send events from both Front-end and Back-end. While the above change is successful in front-end i.e. User ID parameter is being successfully sent as User ID instead of e-mail ID; the same is unsuccessful for backend.For an existing user, backend event with user ID = user_id is NOT showing in AmplitudeFor the same existing user, backend event with user ID = email_id is showing in AmplitudeBackend event with user ID = anything except the user_id is showing in Amplitude.Can you help debug why for existing users I am not being successful for backend events when every parameter passed is the same?We are sending the data directly to "https://api.amplitude.com/2/httpapi"with events 

                        [{
                            'event_type': event_type,
                            'user_id': user_id,
                            'event_properties': event_properties,
                        }]