Propane High Pressure Regulator Leak.

[Picinisco]

With an auto change regulator and both tanks open, what is there to prevent the full tank filling the less than full tank. I just changed my auto regulator since the old one was not drawing gas from the driver side tank when the passenger side tank ran dry. The new regulator, even though set to the passenger side tank, is sending gas to the empty driver side and leaking thru the vent hole in the red pressure regulator on that side. Camping world sold me a regulator that they said would replace the red leaking one. When put back together the new camping world regulator also leaked thru it's vent hole. My original question is why does gas flow in that direction.