I'm working to see if I can connect an external Application LB to an internal passthrough LB in a different project via Private Service Connect. The external LB side shows a good load balancer endpoint, and the publisher project shows a good PSC connection as well. But when I send a test request, I get a 502 with this message in the logs: failed_to_pick_backend
Oddly, if I re-deploy the load balancer with the exact same configuration but non-classic, everything works fine and I can connect to the internal load balancer. I checked the JSON ouptut for each deployment and they're exactly the same other than loadBalancingScheme is EXTERNAL for Classic and EXTERNAL_MANAGED for non-Classic, which is expected.
Are there any special gotchas or configurations needed for Classic? Maybe the protocol or localityLbPolicy parameters need to be set a certain way?