Are you flying on Iberia metal? If so, Plata does give you an extra bag when the regular allowance is 0 or 1 and the overweight fee is waived (not for other OW Rubies, though and only flying on IB, this is not a OW rule). Maybe your booking was not updated if it was made before you achieved status, but it should be OK at the airport (I would bring the card to show at check in if there is a problem).