playing with FGR89, I had Steelix with 1 fighting and 2 metal energies, the damage kept being reduced from the effect of the energy card even though Steelix is already a metal type dealing less than calculated.
metal energy will always reduce your pokemons attack damage by 10, for each metal energy attached to that pokemon. but this also decreases the damage taken by 10 for each metal energy attached to that pokemon. but if the pokemon is a metal type, the first effect is negated and your attacks do the standard damage as printed on the card (like steelix)
not sure why your metal energy wasn’t working correctly, as the energy was working fine last week when i played steelix. Perhaps you or FGR89 had a pokemon in play that did something to the metal energy. But i can only think of Light Dragonite.
I used Unown M 
oh. i see that makes a lot more sense now.
Unown M: Reduces damage done to your pokemon by 30 done to them by opponents metal type pokemon.