Catching up on this discussion. Cross-posting does not work from category feeds unless a given post only appears in at most one category feed. In other words, to work reliably, you can add multiple category feeds (each of which contain different posts) but can’t have the default feed.json.
The reason for this is because of how Micro.blog processes feeds for cross-posting. When a new post is found, Micro.blog checks whether it needs to be cross-posted, and then to make sure it doesn’t accidentally cross-post something twice, it skips processing it if it’s encountered again in another feed. I’ve thought about how to remove this limitation, but for now it’s tricky to use categories for cross-posting.