A company securely connects resources within its VPC to a SaaS solution hosted in the AWS Cloud, utilizing AWS PrivateLink. The SaaS provider's solution is accessed through a PrivateLink endpoint behind their Network Load Balancer (NLB). After adding a new Availability Zone and subnets to the VPC, a network engineer encounters an issue deploying a new interface VPC endpoint for the SaaS solution in the new Availability Zone. What is causing this problem?