401 PermissionDenied error when calling the Azure AI Studio project API

Application with Service Principal ID 8b6a9b11-bbd2-4498-879c-5e8868261d76 is consistently receiving a 401 PermissionDenied error when calling the Azure AI Studio project API, despite having Owner-level permissions on the resource.

Failing API Call: GET https://mspsaturn-ai-central-resource.services.ai.azure.com/api/projects/mspSaturn-AI-Central/assistants/{assistantId}

Example Request ID from a failed call: f6403e5b-f84e-4c12-8cbc-dfeb762438ff

Troubleshooting Steps Already Taken:

Granted the Service Principal (8b6a9b11-...) the following roles directly on the mspsaturn-ai-central-resource scope: Azure AI Developer, Azure AI Account Owner, and AzureML Data Scientist.

Verified roles are correctly assigned via the Azure Portal (IAM) and Azure CLI (az role assignment list).

Attempted to add the Service Principal to the project via the AI Studio UI, but the "Add members" search box does not find service principals; it only finds users.

Confirmed application is using the correct credentials and endpoint URL.

Restarted the application service multiple times and waited over an hour for permission propagation.