Currently in private browsing mode if you log-in to a new site, it asks to save the password. Don't think the browser extension should ask to save logins if you're in private browsing mode. This likely don't need to be a setting, as if you're browsing privately you wouldn't want anything to be saved.
1Password Version: 6.0.2
Extension Version: Not Provided
OS Version: 10.11.3
Sync Type: Not Provided