Done some research over the weekend and I think I found a possible explanation. In my crude drawing below, the system will start to outflow when water level hits point A (lower lip of pipe). As the water rises the flow volume will increase and point B (vent tube opening) will be covered, but siphon action will not start. When water flow hits point C (the upper pipe edge) siphon should start and continue until Point B is exposed to the air, breaking the water seal.
So it is very likely there will be substantial flow through the system without the system going into siphon mode. It should go into full siphon mode only during extreme rain events with large water level fluctuations.
Is my understanding now correct?
