Sorry for the slow reply.
I had a situation where it appeared that documents weren’t expiring in a production environment. My reduced case for testing used expires instead of expiry - the original code was correct.
It turns out the documents are expiring, just at a far far slower rate than they are being created. This should flatten out over time (I hope!).
Thanks again for your clarification @brett19, and thanks for your excellent work on the extension. It is much appreciated.