Yeah that caught me out too, lol.
A full play through of the campaign involves playing both the Black and White side (and then Red afterwards). It's a bit dumb but you basically have to play through the whole campaign twice; once following the Black -> White -> Red route and again going White -> Black -> Red.
Fortunately you only need to play through one of those two routes on hard mode then you can knock it down to normal
Hope that helps!