I was wondering if we should have some sort of exception for banned artists in the case of the image being official promotional art?
This is because the image would be property of the Copyright owner, not just the artists and, the owner has made it freely available.
Like, for instance in the case of the Girls Frontline, or Grandia Online games; howa type 64 being a good example for this. In this case, the image was posted as promotional art on their official website but, we have it hidden from most users, as a banned image.

Of course, if we make an exception, this could get complicated, as some of the official art we have was ripped from inside the games.

I agree, I've found this odd.

While I do find this odd, I feel it's better to play it safe and just leave things be. We don't want to be embroiled in copyright arguments when it comes to banned images lest things get messy.