It's odd because it doesn't work with NodeBB (I woke up this morning missing 4 replies); pulling one should've gotten all of them but I had to pull them one by one (and one of @trwnh@socialhub.activitypub.rocks's still can't make it in).

This is likely related to the problem I described in another thread:

Federated SocialHub Categories

SocialHub admins can federate categories, making categories accessible in the fediverse. This is an overview of current ActivityPub actors that you can follow and participate in from the Fediverse. SocialHub Categor…

SocialHub (socialhub.activitypub.rocks)

> Doesn't this mean Mastodon users et al will no longer get boost notifications when their posts are reshared into a topic?

Correct, only OP will be shared. Unless I am mistaken this is how Lemmy implemented it. Announce-ing activities only, and one single Announce(Note) for compatibility with Mastodon.