I don't run tuned pipes either, but going by what I do know about model engines and engines with expansion chamber pipes, I think your first suspicion may be correct. You do not say what type of fuel and oil content that you are running, and that may have a bearing on what is happening. Randy Smith is the moderator of this section, and the manufacturer of your components, and I'm sure he will see this and answer. He has been out of town at the Joe Nall event but should be catching up on things soon.
In the mean time, I would not run the engine if at all possible, just to make sure the extra oil is not hurting the rod journals or anything else in the top end.
Good luck,
Dan McEntee