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.
You should not use mastodon.social “because it is the largest instance”. The Fediverse was not designed to “join the largest instance”. Instead, go to https://instances.social/ and find something that meets your criteria :)