Unfortunately the issue has cropped up once again.
I’ve made a screencast to show the problem in real time. The cart attribute can be read by the discount function on product pages, but not the checkout page itself with a guest checkout.
I’m going to try an app build/deploy to rule out some deployed vs shopify app dev state behavior. But regardless the run input should be able to read cart attributes in the checkout page too.
Update: rebuilding a deploying the extension makes no difference. The cart attribute isn’t exposed to the checkout guest sessions.