In the Cart module, if you select 'Show first step' in 'When re-entering cart', the cart goes to first step when re-entering the cart, but also whenever the cart is submitted.
If we choose a different payment or shipping method or if we enter a voucher in Step Three, the cart goes back to Step One and forces the user to go through Step One and Step Two again.
I believe selecting this option should take us to the first step only when we re-enter the cart and not when the cart is submitted during the checkout process.
I made a video to show the problem:
https://www.screencast.com/t/JBmc1iB9