Checkout UI Extensions app-data metafields support

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Hi, my team and I were developing the Checkout Extension and discovered that this app-data metafields not work. Is there any way I can access this value? If not, I was wondering if this issue is on your development plan?
BTW, I noticed a tip in the unstable version of the Shopify dev docs told that "Metafields on an app reserved namespace cannot be accessed by Checkout UI extensions." However, I did not find this tip in other versions (including the latest), which might cause confusion and people misdirection that it is enable in the stable version. Please be aware of this.