Twenty Seventeen and WP 4.7 was the first time starter content was introduced. This allows the theme to show you sample data for how the theme can look. It is only shown on a fresh install, but it should only be in Customizer and only saved if you save it. Twenty Seventeen also has a setting for how the static front page is shown, as you can choose to show other pages on the front page. I’m not sure if this might be overriding your page template.