PS. My VM has 2 network interfaces. IAP is connected to secondary nic, but --verbosity=debug output from gcloud compute start-iap-tunnel contains somewhat strange output:
INFO: [0] Connecting with URL ['wss://tunnel.cloudproxy.app/v4/connect?project=XXX&port=22&newWebsocket=True&zone=asia-northeast1-b&instance=XXXXXX&interface=nic0']
Probably start-iap-tunnel should allow passing --interface=nic1