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.
no.
If you would follow the person then all their content that they post afterwards should reach your instance. So each instance only gets the information that is needed to show the content that people have subscribed to. Otherwise each server would have to download and store ALL information which is posted, that would be unpractical.
So to view a person that you don’t yet follow, go to their original profile. If you follow them you can trust that all their content will reach you from that moment (if they or their server isn’t being blocked by your server, but that’s pretty rare)