An antique car restoration shop near me says that the alcohol in today's fuel will dissolve the varnish (old gas sediment) on the bottom of your fuel tank. That junk will tend to release all the other sediment that was at one time stuck to the bottom of the tank. He had several examples to show me of the volume of debris that comes out of the older tanks. Secondly, the alcohol dissolves the inner surfaces of the fuel lines and the fuel pump diaphragm. This also sends junk through the fuel system and into the fuel filter. If the filter is located downstream of the pump, you may have debris blocking the valve partially open in the pump which would allow back flow away from the engine. The volume from the pump could be sufficient to overcome the back flow leakage during engine operation, but not after sitting for a bit. Lastly, alcohol absorbs water and your gas tank will rust faster.
Just a guess, it's your car and I can't really see what you have.