Coupon code textbox on checkout page not submitting properly

[noisecapella]

Steps to Reproduce

Add a valid coupon code via the checkout page. Refresh page. Just click the 'Apply' button again. It should appear to do nothing since the coupon is already applied, but instead the frontend submits an empty coupon code to the basket which erases it. I think there's probably some faulty logic where the coupon is shown in the textbox but it's not actually in the component state.