The matal washer you placed inside the tank is, or will be, the leaker. You have contact between too metal, not so smooth surfaces....the top of the washer and the underside of the bolt head. That joint cannot seal out water. Inside the tank should be the head of the bolt, to rubber washer...that's it.
But the description that it leaks after a flush points to something on the tank-to-bowl gasket. Maybe disassemble and take a look at everything