A copper sealing washer works by being softer than the two faces it sits between. Tighten the bolt and the copper flows into the tiny marks on each face and closes them off.
That only happens if the copper is soft. Copper hardens as it is worked, and sheet can come off the rolling mill hard enough that the washer behaves more like a thin steel ring. It takes the load, it does not flow, and it weeps.
That is why copper sealing washers are annealed, heated and cooled to soften them. It is also why a reused copper washer so often leaks. It was work hardened the first time it was tightened.
Two things for anyone buying them. Ask whether the washers are supplied annealed, and put it on the drawing if it matters. And do not reuse them on anything that holds pressure, whatever the fitter says about how good the old one still looks.