When subscribing to a community on another instance, the subscribe button will change to “Subscribe pending”.
This appears to be a bug, as the content you’re subscribed to will appear anyway.
You can follow this issue in the issue tracker, as well.
#LemmyAdmin
You’re right. I think, this only applies to lemmy.ml. Maybe they’re using a whitelist. But I can browse (and I think even recieve updates for) these communities just fine.