I worked out why this feature of our API doesn't work at all the way the documentation says it does.
On the minus side, making it actually do what it says on the tin may ... actually be impossible with the data we currently have? Specifically, the data structure for "subscriptions of mine, sorted by date" is a datasource we have, but it's not clear that "people who subscribed to me, sorted by date" is.
But at least now I know that I'm not holding it wrong: it is just broken.
