This is a Shopify Checkout API and Draft Order API limitation.

You can refer to their documentations here:

https://shopify.dev/docs/admin-api/rest/reference/sales-channels/checkout

It is also documented that Shopify Discount codes might not work for accelerated checkouts as mentioned in the reply by Shopify to a question in their forums in which their solution is to hide the express checkout buttons:

https://community.shopify.com/c/Payments-Shipping-Fulfillment/Coupons-not-working/td-p/680827