it returns the oldest posts first, which is the opposite of what other software are doing (but I’m not sure how much of an issue that actually is)
it does not link to previous/next page, so only 12 posts may be accessible. I’m not very familiar with the Activitypub crate, but I guess there is something about using OrderedCollectionPage instead/in addition to OrderedCollection
For ActivityPub, you need to have both an OrderedCollection on /@/USER/outbox, with a first field, containing a link to the first page of the outbox, and an endpoint like /@/USER/outbox/page=PAGE serving a specific OrderedCollectionPage, that should have the items (an array of activities), prev (a link to the previous page, if any) and next (a link to the next page, if any) fields.
I don’t know if that’s clear, I can try to explain it in another way if you want.