Hello,
I suppose your issue appeared after WooCommerce plugin update, not after theme update because once we enabled the official WooCommerce theme Storefront we got the same problem https://gyazo.com/72807306f61431fc486fb09d779a20b8
Probably some of the third-party plugins, for example, WooCommerce Checkout Field Editor may cause the issue. Try to disable plugins and check. Once you detect which one is the reason of the problem contact plugin author to the issue.
Theme support can do nothing if your checkout does not work even with the default theme because of third-party plugin.
Regards