The method handle.abandon doesn't actually abandon the request. Instead it returns a Completable which abandons the request once it is subscribed. Unfortunately, the code above ignores the returned Completable. Even more unfortunate is that fixing the code above triggers multiple failures in the ProxyBackendTestCase, so the bug is a little more complex that it first appears. Yannick has discovered other related problems in our load balancer implementations.
I'm not sure how severe this issue is as it appears no one has noticed the problem in practice.