A long time ago, I had a mstdn.social account and was trying to view a friend’s content on like mastodon.art. I couldn’t see his content via mstdn.social at all, but I could if I just went to mastodon.art. Mastodon.art was not in the list of servers mstdn.social did not federate with. I assumed this was a glitch.
This was precisely what was not working. I could follow them, but no content would appear iirc.