Simple Function (NodeJS) - Webhooks not firing for private add-on

I am hardening an integration for my business using a Private Simple Function (UUID: 1afce2dd-e6c3-4c40-945c-23a2643a7a2b).

Current Setup:

Manifest (v1.0.2): Includes scopes: read_invoices and subscriptions: [{ "object": "invoice", "event": "sent" }, { "object": "job", "event": "created" }].

Status: Add-on is ON and authorized via the Private Install URL.

Script: Standard ES5 https POST to a Zapier webhook.

The Problem: Despite being authorized, the Function Event Log remains empty. I have tested creating new Work Orders and sending Approved Invoices, but the exports.handler is never triggered. Even global-scope console.log calls intended to run on script load do not appear in the log.

Question: Is there a known issue with event routing for private add-ons, or is invoice.sent the incorrect subscription syntax for the Simple Functions environment?