It's not for the developer to decide any more than the browser. It's up to the user.
So if autocomplete breaks functionality then there's nothing the user can do if the developer assumed autocomplete='off' would continue to work.
Except disable autocomplete on every site, which is kind of overkill.
Developers should not expect that things specced as hints will be treated as authoritative direction and should not make software that breaks if that unreasonable expectation doesn't hold.