I believe the dlc download will create a folder without the necessary rpa file? it's seems to be entirely empty actually. At least on mac anyway! I wonder if that might be the problem people are so commonly running into? Though I'm not the most tech savvy so I could've done something wrong on my end^^;;
Edit just download the dlc from the itch website instead of the app and put the file in the game urself and it should work fine! I managed to figure it out anyway eheh