Every Evolution I've had, does this when new.

I've pulled the back plate on each and the only thing I've seen is a bit of wear where the piston rod rubs against the backplate.

All of the Evolutions I've had, do this as those owned by other club members.

Once the engine is broken in this very very slight contact stops.

I'd imagine you could cure this with another back plate washer in place, but it does stop by itself after a few gallons.