Template submission rejected: "We couldn't verify this template from the repository"

I'm submitting https://github.com/spree/storefront (public, MIT) to the Templates Marketplace. Every submit fails with "We couldn't verify this template from the repository. Check that the selected branch and directory contain the template source code." The API returns 422 with no field errors.

GitHub

GitHub - spree/storefront: Open-source Next.js eCommerce storefront for @spree. DTC & B2B, Stripe/Adyen/PayPal payments, multi-region, one page checkout. MIT license.

Open-source Next.js eCommerce storefront for @spree. DTC & B2B, Stripe/Adyen/PayPal payments, multi-region, one page checkout. MIT license. - spree/storefront

Branch: main. Path: empty (the Next.js app is at the repo root; package.json with next is at the root)
README starts with a level-1 heading and includes the Deploy button
Demo: https://storefront-spree-commerce.vercel.app (production deployment of a Vercel project connected to spree/storefront, publicly accessible)
I reconnected GitHub in Account Settings and resubmitted; same error

Could you tell me which check is failing?

GitHub

GitHub - spree/storefront: Open-source Next.js eCommerce storefront for @spree. DTC & B2B, Stripe/Adyen/PayPal payments, multi-region, one page checkout. MIT license.

Open-source Next.js eCommerce storefront for @spree. DTC & B2B, Stripe/Adyen/PayPal payments, multi-region, one page checkout. MIT license. - spree/storefront

Branch: main. Path: empty (the Next.js app is at the repo root; package.json with next is at the root)
README starts with a level-1 heading and includes the Deploy button
Demo: https://storefront-spree-commerce.vercel.app (production deployment of a Vercel project connected to spree/storefront, publicly accessible)
I reconnected GitHub in Account Settings and resubmitted; same error

Could you tell me which check is failing?