Dev Dashboard client-credentials returns application_cannot_be_found

Link to Shopify Store

Reproduction steps

Created a development store tb-ai-staging-dev.myshopify.com in the Shopify Dev Dashboard.
Created Dev Dashboard app TB AI Staging in the same organization.
Configured the app with only the read_products scope.
Released the app version and installed it on TB AI Staging Dev.
Confirmed the app shows 1 current install on that development store.
Sent a raw client-credentials request:
POST https://tb-ai-staging-dev.myshopify.com/admin/oauth/access_token
Content-Type: application/x-www-form-urlencoded
Body: grant_type=client_credentials client_id= client_secret=
Shopify returns HTTP 400 application_cannot_be_found.
Shopify Support recommended a clean uninstall/reinstall. I did that, then ran a new uncached raw authentication request. The same error still occurs

Additional info

Shopify Support confirmed that the app and store configuration appears correct and recommended posting here for developer-level investigation.
App: TB AI Staging
Development store: tb-ai-staging-dev.myshopify.com
Scope: read_products
App and store are under the same Dev Dashboard organization.
The app is installed on the development store.
A clean uninstall/reinstall did not fix the issue.
Latest fresh Shopify request ID: 61606fa6-551f-415c-ac69-c299e9d55a38-1789475633
Earlier request IDs: c470a726-39e2-45b5-8227-4d87b805dcf7-1789444020
21460211-141f-482e-beca-7467dfc93554-1789440061
Could Shopify developer staff inspect the token-service/backend association for this app and development store and explain why the client ID is not being recognized?
I can provide additional non-secret diagnostics if needed. I will not post the client secret publicly.

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