I’m using Sage 8.4.2 theme. After upgraded to WooCommerce 3.3.5, if I go to checkout and choose PayPal as the payment method, it doesn’t bring me to the PayPal page and instead goes straight to the thankyou page. I’m using the PayPal Standard payment gateway that comes with WooCommerce.
The WooCommerce support has suggested me to temporarily switch my Sage theme back to TwentySeventeen one. Doing the test, the issue has been resolved, so the problem is in a conflict between Sage and WC3.