After configuring Cloud NAT one instance still uses its external IP for outbound traffic. What is the most likely cause?

Cloud NAT provides outbound internet connectivity for instances without external IP addresses. If an instance does have an external IP address configured, it will use that external IP for outbound traffic by default, bypassing Cloud NAT. This is the most common reason an instance wouldn't use Cloud NAT despite it being configured. Multiple interfaces are not inherently an issue, as Cloud NAT is configured per subnet. Static routes using RFC1918 ranges are internal and would not affect outbound internet traffic. An instance being accessible by a load balancer's external IP address only affects inbound traffic to the instance, not its outbound traffic.