Ok, I confirmed my hypothesis.
So since RDW does not trigger (there is a 60 sec acceptable HB pause) it keeps sending Heartbeat messages, and eventually it gets a HeartbeatRsp from the new system containing a new UID. Then it rewatches the actorrefs which causes Terminated coming from the new system since the actorref points to nothing (actor UID does not match). 

This does not invalidate the test though since it tests remoting sysmsg buffer reset, but that pesky Terminated can come at unexpected times. For me it works if I add the expectTerminated before the Identify pinging but I needed a 10 second timeout to make it work. Maybe we should add an unwatch.