A company operates hundreds of Amazon EC2 instances across two production VPCs (VPC A and VPC B) in all Availability Zones of the us-east-1 Region. To comply with a new security regulation, all traffic between the production VPCs must be inspected before reaching its destination. The company has implemented a shared VPC containing a stateful firewall appliance and a transit gateway with VPC attachments to route traffic between VPC A and VPC B through the firewall for inspection. However, during testing, the transit gateway drops traffic when it traverses between two Availability Zones. What should a network engineer do to resolve this issue with minimal management overhead?