They appear in the server connect file, as well as in the data bindings in the front end server connect component, however they do not output to the front end where they can be displayed in a repeat.
How to reproduce
Create a server connect file with Retrieve Checkout Session. Call this file from a server connect on the front end. Set a table body to repeat using the line items from the data picker.
If it's working for ya, then don't fix what's not broken. Your approach probably lets you 'repeat' through all of the key value pairs while the Wappler approach doesn't out of the box...even though I should be able to treat it like a JSON object.
However, for people who are more comfortable using the native Wappler elements within their server connect API's, the "List Checkout Session Line Items' will work. Stripe's relatively new CLI, offered on Stripe's website while you are logged into your account, let's you query transactions and checkout sessions easily, listing all key value pairs so you can quickly see what's populated.