Just found a browser related bug with the default frontend layout (not 3.1 related). Chromium browsers (not sure if others as well, webkit works) don't seem to submit the

params[:checkout]

parameter when clicking proceeding to checkout from

button_tag class: 'button checkout primary', id: 'checkout-link', name: 'checkout'

. Setting an explicit value fixes the issue. https://github.com/codegestalt/solidus/commit/8e1aaa443db2a0bfb389ebc64ed6c45d68f49033

Thanks for reporting, it seems more a Rails-related problem, the button tag comes from there

You're right. Seems to be Turbo related.