For what it's worth your blog does show up fine in NodeBB as well. Perhaps you are missing the @context property and so Mastodon is refusing to parse it?
The mirroring functionality was better suited for NodeBB before it supported remote categories.
It's a little redundant now but still has value if the topics are cross-posted instead of moved.
Unfortunately cross posting isn't quite available yet (nor have I thought through the various aspects of it).
My recommendation for now is to pause category synchronization and use remote categories primarily.