When using the "default" network/VPC it is not possible to select a different subnetwork than the default one (see https://github.com/jenkinsci/google-compute-engine-plugin/blob/aff330dda6269ad3cd35bbb4dde55638dcb3eb9d/src/main/java/com/google/jenkins/plugins/computeengine/AutofilledNetworkConfiguration.java#L88).

Is there a reason for this behavior? In my case it would help if I could select a subnetwork I created in the default VPC.