Deleted Outlook events not triggering notifications, sometimes just slowly

Question

I'm using this Node.js sample application provided by Microsoft to subscribe to change notifications on user calendars. I'm able to get it working, however not very reliably. It seems that deletions are the most unreliable, but I believe I have seen issues with other event creations and updates as well (I've been working on this issue for a while, so I lost track).

Today however, I did a simple test where I started up the app, and created a subscription on my own calendar. Once the subscription was created, I created an event, and received a notification. I then updated the event, and received another notification. I then deleted the event however, and did not receive a notification. I waited for a little over an hour to see if it would eventually show up, and never did. I have seem them come through slowly, and not come through at all.

I have the "changeType" for the subscription set to "created,updated,deleted", so that is not the issue. Also, it is not just an issue with the UI of the sample application (although the UI on the sample app does not work with deletes either, it receives an error when the delete notifications do come through). I am able to see when the delete notifications come through using the ngrok HTTP inspection (http://127.0.0.1:4040/inspect/http). This shows all HTTP requests coming through to my webhook, and I am checking this for the delete notifications instead of the UI of the sample app.