Despite being Psychic/Steel, Metagross(and its prior forms) is taking 2x damage from Dark/Ghost even though it should be 1x. There was an earlier post about how Metagross is also being affected by Poison type moves. It seems that some of the resistances of Steel got overlooked or something.
I took a look at the 8.3.3 changelog just in case but it doesn't seem that this was fixed yet. The wiki also incorrectly lists Dark/Ghost dealing 2x.
Have not done specific testing, but it does seem to be getting stab bonus on Steel moves despite the other issues.