Any update on this
Topic summary
Main issue: Creating a metafield definition via the Admin GraphQL API (metafieldDefinitionCreate) failed when metafield values already existed for the same namespace/key on PRODUCT, returning: “Namespace and key is already in use for a set of your metafields.”
Context: In the Shopify Admin UI, users could attach a definition to existing metafields via “Metafields without a definition,” apparently using a forceCreate flag. That flag was not accepted by GraphQL, creating inconsistency between UI and API.
Community feedback: Multiple developers confirmed the same error and asked for parity and an explanation for the differing behavior.
Latest update: A participant reports that as of this week the behavior now works as expected. You can create a Metafield Definition after values already exist for that namespace/key, with no error.
Outcome/Status: Appears resolved with API behavior now matching the Admin UI. No official Shopify staff confirmation or changelog cited in the thread; timing and exact change not documented there.