Terraform Provider 1.18.4 fails on Security Profile IAM binding - 403 Forbidden

Context:
Running automated deployment pipeline via GitHub Actions.
Environment: Genesys Cloud Org (EU-West)
Terraform Provider: 1.18.4
Genesys Cloud CLI: 2.4.1

Attempting to assign a custom IAM security profile to a user resource. The security profile exists and is valid. Manual assignment via UI succeeds immediately. Terraform apply fails consistently at the user resource block.

Error output:

Error: Error updating User "12345": 403 Forbidden
Message: "Insufficient privileges to perform this action."

The service account used for Terraform has admin role and security-profile:write permissions verified via genesyscloud_auth_service_account debug logs. The profile ID is correct.

resource "genesyscloud_user" "agent" {
 name = "Test Agent"
 email = "[email protected]"
 security_profile_ids = [var.custom_profile_id]
}

Relevant API call seems to be hitting /api/v2/users/{userId} but returns 403 specifically when the securityProfileIds array is populated.

Question:
Is there a known bug in provider 1.18.4 regarding security profile binding? Or is there a specific permission grant required beyond standard admin roles for programmatic IAM updates? Looking for workarounds or confirmation if this is a backend API restriction.