Hi! I think that no "local" user had subscribed to those communities, so this instances had not fetched those posts.

However... As soon as you fetched the community, the first 10 posts should have been automatically fetched. But you are right, the posts were not fetched as they usually are. Future posts should be fetched. My suspicion is that this could be related to the new federation queue system, but I am not sure. I also don't know if this is affecting only how posts made before the update are fetched, or if this will also affect newer posts.