The pressure created by the vapors may be pushing the fuel back towards the tank and the vacuum created by the pump may not be enough to overcome the pressure.

Gasoline isn't what it used to be and it goes to vapor easier nowdays. When fuel boils in the carb it can cause the floats to sink and you could get a rich condition. I've read somewhere that parts of the fuel can start to boil at 100 degrees.

I think you need more space between your fuel line and the heat barrier which is why I suggested the pipe insulation and the heat barrier on top of that. The heat barrier by itself probably would work for small amounts of time but I'm thinking the engine compartments's constant heat saturates everything quickly and since the heat barrier is touching the fuel line it ends up being a conductor of heat.

Foam insulation doesn't conduct heat to well.

Keep in mind that I just trying to give you ideas as I haven't tried any of these ideas yet.