OK, so when you use the internal domain, traffic is routing directly between two app instance. It's not going through Gorouter, so you lose the load balancing that is provided by Gorouter.
Without Gorouter the only load balancing you get is at the DNS layer. When you resolve `dummyservice.apps.internal`, the response should be round-robined.
Ex:
vcap@c1a47cdf-f581-49c4-771c-250d:~$ host speed-test.apps.internal
speed-test.apps.internal has address 10.248.39.92
speed-test.apps.internal has address 10.252.251.126
speed-test.apps.internal has address 10.253.200.126
vcap@c1a47cdf-f581-49c4-771c-250d:~$ host speed-test.apps.internal
speed-test.apps.internal has address 10.252.251.126
speed-test.apps.internal has address 10.248.39.92
speed-test.apps.internal has address 10.253.200.126
vcap@c1a47cdf-f581-49c4-771c-250d:~$ host speed-test.apps.internal
speed-test.apps.internal has address 10.252.251.126
speed-test.apps.internal has address 10.253.200.126
speed-test.apps.internal has address 10.248.39.92
See how the .92 address rotates through the response.
This could be a problem though if your app is caching DNS or if your app is caching connections to a backend service. I believe that the Java buildpack will disable caching by default (check your staging output, it emits a message saying when it disables this). If you can confirm that DNS caching has been disabled in the JVM, then the other culprit could be that connection reuse is happening.
Many apps will keep HTTP connections open and use them multiple times for different requests. This saves the overhead of making a new TCP connection & establishing a new TLS session (when HTTPS is used). The problem is that this is going to be a connection to a specific backend, and that won't change until the connection is dropped. This could lead to unbalanced traffic loads to your backend apps.
To work around this, you need to have a smart client that can load balance. Check out the LoadBalancerClientFilter or ReactiveLoadBalancerClientFilter
in SCG. I believe that should do what you want.