@[email protected] I think I know why this is the case.
For you on Hubzilla, the message as received by NodeBB contains the following collection https://community.nodebb.org/uid/2/followers. The expectation according to the spec states that I must forward that message along to those users. This is so those users would get the whole conversation, and avoid ghost replies, etc.
NodeBB does not do that at the moment. Thanks for letting me know about it.
The thing is, while Hubzilla does this, and even if NodeBB implemented it, Mastodon does not send those collections. When a reply is received, its recipients list usually contains only the person being replied to, the author's follower collection, and anybody else who happens to be mentioned.
So that's the reason why you can't see all of the replies in the thread, because another implementor didn't send them to everybody 😄
It's actually quite interesting that Hubzilla explicitly includes OP's follower collection in the recipient list, I was thinking of doing the same for NodeBB.