You are creating a Cloud Storage bucket in project prj-b that is encrypted with a Customer-Managed Encryption Key (CMEK). The key is stored in Cloud KMS in project prj-a, is backed by a Cloud HSM, and is located in the region europe-west3. The storage bucket is being created in the region europe-west1. You are unable to access the key during bucket creation and need to identify the root cause of the access issue.

What is the most likely cause of the problem?