Man, had similar work trouble but not as terrible as what Miwa experienced, but working in production house is basically giving all the credit of your hard work to higher up.
i once did sets of 6 character design for niche auto battler gacha game, each with 3 level up iteration, the client like it, ask for more, even ask for direct design brief with me instead of the usual three way communication via production manager. Did all the stuff from drafting, sketch, lineart, to color and render solo with no pass over or swap with co-worker.
But once the character been released, not one credit given, and i was paid less than $300 for all of it.
Left the production house, and when i left, i got told that the place is basically a black company and was infamous, but no one really told be before the fact
Fortunately i don't get harrased or verbally abused like Miwa experience in the story, but it just an overall shitty place with shitty paid.
Now i just work as freelance, the money is honestly better once you hit the market right