Hi Andre,
I was facing the same issue with 1.12-rc2 version, I was using custom network(overlay) and creating a service with “–network” option, the requests were routed to the same service instance.
Then, I have removed the services and custom network, and created services without network options(default network, ingress), and then it started routing request to two services in round robin fashion.
Not sure if you have to do anything else to get it work with custom overlay network.
Note - I am using nginx in front of services(1 instance of nginx, 2 instances of web service)
Cheers,
Anil