Hello,
@kyratn - Thanks for the help
@joris85 - As kyratn mentionned, the setting became deprecated and is only used when you have the "legacy add to cart".
When using the cart system, the plugin "cart notification" is doing the job of the previous setting.
If the plugin is published and configured, it will display a notification and the customer won't be redirected to the checkout.
If no notification is displayed, the fallback system will redirect the customer to the checkout, so he will know that his product has been added to the cart.
Please be sure to be up to date ; your "cart notification" plugin looks not up-to-date regarding some missing options (in 3.0.1, the plugin can display a "tooltip" or a "popup").
If you have some javascript errors in your page ; the plugin script might not be initialized and it could have some side effects.
Regards,