Do you get emails from Amazon when your orders are delivered? It’s based solely on email tracking, so if there are no emails, the status won’t update.
I have the same problem, but that’s because Amazon never confirms delivery for me via email. Most of the things I order from other online retailers don’t either, but at least they ship via services like UPS and Canada Post that do provide delivery notices, and the new feature is surprisingly good at associating those with the original order (likely just using the tracking number).
The more annoying thing for me is that it seems to pick up recurring subscription email receipts as orders, even for things like Apple One and AppleCare+. That’s a lot of clutter for no real benefit.