In your Child theme, do you by chance have any template parts like
content-list.php left over from the beta version of Jump Start?
And on a sidenote, for your “blog” page, if you’re giving it the Post List page template, you shouldn’t be setting it as your “posts” page in your WP > Settings. So, you should go back to that settings and just leave the “posts” page blank. When you do that, WordPress is taking that page and replacing it with the theme’s homepage file, generally index.php, which is not what you want.