Based on this comment:
There are some anomalies in the federation between Lemmy and NodeBB. In the past, I noticed that Lemmy only federated communities/categories where at least one Lemmy user had started a conversation. Yes, I know it seems like magic, but I imagine there is a reason for it…
If what the comment said is accurate, it would be an issue if I want to subscribe to a category where I"m not supposed to create a new topic.
My guess is that I couldn’t see them because the OPs are from Mastodon-like accounts.This might be the reason. Lemmy has limited support for Mastodon-style Note objects as top-level posts.