Application A runs on Amazon EC2 instances behind a Network Load Balancer (NLB).

Comprehensive and Detailed Explanation From Exact Extract of AWS CloudOps Doocuments:

VPC Flow Logs show the request arriving and being ACCEPTed on dstport 8080 and the corresponding response being REJECTed on the return path to the client’s ephemeral port (59003). AWS networking guidance states that security groups are stateful (return traffic is automatically allowed) while network ACLs are stateless and require explicit inbound and outbound rules for both directions. CloudOps operational guidance for VPC networking further notes that when you allow an inbound request (for example, TCP 8080) through a subnet’s network ACL, you must also allow the outbound ephemeral port range (typically 1024–65535) for the response traffic; otherwise, the return packets are dropped and appear as REJECT in flow logs. The observed pattern—request accepted to 8080, response rejected to 59003—matches a missing outbound ephemeral-range allow on the subnet’s NACL. Therefore, the cause is the subnet NACL, not security groups or on-premises ACLs. The remediation is to add an outbound ALLOW rule on the NACL for the appropriate ephemeral TCP port range back to the on-premises CIDR (and the corresponding inbound rule if asymmetric).