Remote Event Receiver not firing when created using App Context

[dgtheninja]

When I create a remote event receiver using an App Context the endpoint doesn't get called, however when I create the event receiver with a user context it does get called, why is this?

I can list the event receivers and the one I have created is there and looks correct

Example Code
var am = new AuthenticationManager();
using (var cc = am.GetAzureADAppOnlyAuthenticatedContext(siteUrl, ClientId, Tenant, CertificatePath, CertificatePassword))
{
cc.Web.AddRemoteEventReceiver("ListAddedFunction", "https://redacted.ngrok.io/api/ListAddedFunction", EventReceiverType.ListAdded, EventReceiverSynchronization.Asynchronous, false);
}

If I replace the login with
using (var cc = authManager.GetWebLoginClientContext(siteUrl))
then the endpoint is called as expected