Having just updated my Wordpress/Civi site to current versions 5.5.3 and 5.31.1 respectively I've been checking the site to ensure it's all still working as expected. On a membership sign-up form that is being displayed via a shortcode I notice that when the user clicks to submit the form, rather than seeing the confirmation page where they can check the information they have entered prior to making their payment at Paypal, they are seeing a page which just shows the shortcode.
As a logged-in admin user I get the same thing. If I change the configuration of the contribution page to turn off the confirmation page, the form then works as expected. If I turn the confirmation page back on and try again, as a logged-in user the confirmation page is displayed correctly, but if I try it in a different browser as an anonymous user, I still see just the shortcode.
The workaround is to disable the confirmation page, but it's less than ideal.
Looking at the browser console, when the form is inserted via shortcode, the resulting page has a string of JS errors of the form : "SyntaxError: Unexpected token '<'" which don't arise when viewing the contribution page itself (i.e. using the Civi URL). I don't know whether this is what's making the form break in this way.