Hi,
Thank you for the message and I'm really sorry for the trouble.
The DLC has been checked by Steam before release and it's supposed to work. Though I couldn't properly check it myself since Steam's system, especially for DLCs, is chaotic. It's hard to find where and how to check these things. I will try to check again and fix it.
If you want, I can send you a code that gives you the artbook on itch.io. Everything seems to be working perfectly here. Just send me an email through the website: https://raikonkitsune.wordpress.com/
- Raikon