Does the owner of a public Instagram account bear sin if their followers access the accounts of those they follow and find therein that which is forbidden?

It does not harm you if someone enters your page and disobeys Allah by looking at followers, for the responsibility is upon them, and a person is not obligated to restrict followers. Allah Almighty says: (Rather, man will be a witness against himself), and (And no bearer of burdens will bear the burden of another). There is no harm in drawing that which has no soul and earning a living from it.