Cart/Checkout Sales Channel Attribution Not Working

[fuzzy]

I'm following the steps in 

https://shopify.dev/apps/channels/cart-permalinks/cart-permalinks#step-3-attribute-an-order-to-a-sal... to set up attribution for a Shopify Sales Channel app that I built. According to the instructions, I should be able to append my access token to the cart URL I generated like:

http://{shop}.myshopify.com/cart/?access_token=#{storefront_access_token}

and sales that are created from that cart should be attributed to that sales channel. No matter what I pass in, the sales are still getting attributed to "Online Store" (as seen on the order page in my shop's admin dashboard).

Did something change on how to do sales attribution? I'm fairly confident that this has worked for me in the past. To deeper test, I tried creating a discount code that was *not* synced to the sales channel whose storefront access token I'm passing in, and the checkout still accepts the discount code. So it seems that this `access_token` parameter is being ignored.

[VO1D]

Did you find any solution for this?

[fuzzy]

Unfortunately I never did. Please let me know if you find anything in your search.