water don't run uphill, and neither does gas.
There is only one way for the gas to actually return to the tank, and that is if the tank is not venting, and develops a vacuum.
More likely is that the pump is sucking air.
Or that after the engine is shut off, the fuel is boiling in the line, when the float drops..... due to it boiling in the bowl. Then possibly the vapor pressure in the hot tank, forces fuel up the line and it boils in the pump, and the fumes escape thru the now continuously open float-valve.... Next morning the tank has cooled off, the vapor contracted,and it pulled the fuel back.
IDK I like the story.
Check your venting.
