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.
If you type their “@username@domain.tld” in the search field on your instance, their account should pop up. Once you follow them from there, they’ll be in your feed. I’m not aware of any specific bug with this.
This was precisely what was not working. I could follow them, but no content would appear iirc.