The SalesforceDX Namespace Registry Connected App is used during the "Link Namespace" step of the Linking Namespace Registry.
After PKCE rollout "SalesforceDX Namespace Registry" app has PKCE enforcement enabled, but the Link Namespace flow does not send the required PKCE code challenge, causing the login to fail with missing required code challenge. This affects namespace linking.
Reach out to Salesforce Support with the details.
005408535

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