The problem is in the fuel return system.
Interestingly the tank shouldn't pressurise as you are using fuel to run the engine so it should be coming under lower vacuum.
I'd first disconnect the fuel return line at the engine and apply compressed air to it to ensure it isn't blocked between the regulator and tank.
If that is clear I would look at a faulty fuel pressure regulator.
If the FPR is faulty the engine would see too higher pressure and run rich.