Here is the short answer; I will post a more detailed answer later, along with a diagram.
The Zongshen RX3 employs an oil separator in the crankcase vent tube, which is the same design employed by my Honda XR650L motorcycle. The difference in the two designs is that the oil separator for the XR650L allows the liquid collected from the oil separator to be sent back to the crankcase, and the oil separator for the RX3 collects the liquid in the descending tube.
Some combustion gas, along with a trace of fuel, always gets past the piston rings and enters the crankcase; that is why every crankcase has a vent. Most engines vent this contaminated vapor either directly to the ground, or to the airbox, where the contaminated mist is introduced to the fuel/air mixture which is burned in the combustion chamber.
Some owners of the XR650L remove the oil separator, extend the vent hose to a higher location, and close the end of the vent hose with a small air filter. These filters always accumulate a collection of contamined goop which has been expelled by the vent hose, so the filter must be replaced periodically. The stock oil separator of the XR650L merely returns the collected oil, and the contaminants, back to the oil in the crankcase.
The contaminants collected in the downward vent tube of the RX3 will contain a little gaseoline. However, this contaminated liquid is normal, and it is not pure fuel. Instead of returning this contaminated liquid to the crankcase, the RX3 is designed for you to collect it, and dispose of it as you would with used motor oil.
If you fill the crankcase with too much oil, you will also find oil in the vent tube, along with the contaminated liquid with contains fuel/goop from the combustion process. If you don't overfill your engine oil, you will just get the contaminated liquid, which contains a trace amount of fuel which escaped past the piston rings.
If you get concentrated fuel in this collection tube, the fuel injector is probably dumping extra fuel into combustion chamber, which is leaking past the piston rings, and ending up in the descending vent tube. However, if the collected liquid merely smells a little bit like fuel, this is normal, and you should not be concerned.