Comments don't show up until there's a user from the local instance subscribing to the remote instance. You need just one subscriber though.
Think of it like email subscription where you get emails from the moment you subbed.
On Mastodon it is possible to search for a comment's url and have it fetched manually, but here that doesn't seem to be the case yet, although it surely will be implemented.
The good thing is that it can be automated.
On the flip side it works that way to reduce the load. Makes no sense in getting all comments if no one is going to read them. But I agree that once a user clicks on a thread, comments need to be fetched from the remote server.
Hm, I have to ask again: initially thought I had to follow a user on the remote instance, similar to mastodon. But is that possible in Lemmy? See it only in kbin.
Because the remote community in question I have subscribed to before posting.
Ok, got it. In this case it was something else: on my local instance it showed that I subscribed to the community, but on the community page itself it was stuck in status pending. Subscribed again now, hopefully this solves the problem.