A company has stateful security appliances deployed across multiple Availability Zones within a centralized shared services VPC. The AWS environment includes a transit gateway connected to both application VPCs and the shared services VPC. Workloads in the application VPCs are deployed in private subnets spanning multiple Availability Zones, and all east-west (VPC-to-VPC) traffic is inspected by the stateful appliances in the shared services VPC. Users report dropped inter-VPC traffic between workloads in different Availability Zones, confirmed by ICMP pings. The network engineer has eliminated security groups, stateful device configurations, and network ACLs as potential causes. What is the reason for the dropped traffic?