google cloud platform – GCP Compute Engine outbound traffic to flow through the load balancer to internet instead of flowing directly

I have given my Load balancer IP and SSL public cert to be whitelisted to an external company to access their APIs thinking the traffic will flow through the Load balancer IP. But the traffic flows to the internet directly from the VM. Could you please help how to resolve the issue, as it is not ideal to whitelist all the VMs’ IP addresses.

Also the client is using mTlS and I’m using GCP Load balancer TLS termination with GCP managed SSL certificated which adds to the problem, As I’m unable to get the private key for certificate from GCP to terminate the tls in the application instead of Load Balancer.

No Ideas to resolve yet.