Getting the year wrong on a bicycle is not a big deal, for most people. It may have been an honest mistake. IMO, a mid-1990s high end Bianchi is going to be better performing and have comparable value to an equivalent 1986 model. So unless, the seller also misrepresented it as being something rare, like an X-4, or misrepresented the tubeset or components, I'd leave as is and enjoy the bicycle.