Japanese, living here. I'd heard 人質司法 (hostage justice) used in news commentary but never really pictured what it looked like inside. 5-day showers, food slid through a slot, sleeping on the floor with lights on. None of that is what most people here imagine when they hear "detention." A lot of us live with this background feeling that "if you get arrested here, you're done" even if you didn't do anything. Part of it…
You might find her explanation of why she was detained interesting: https://youtu.be/Q2epTf2IW1g?si=ipy4m3rgFDw3b3cD
I’m guessing either she didn’t understand the warnings, or she didn’t follow their guidance.
Simple example, they may have asked her to follow a procedure before leaving the country, and she didn’t because she “thought it was over”.
The law enforcement machine in Japan doesn’t like to arrest people. 99% of the time or so, it only arrests when they have an open-and-shut case and/or the person had been warned multiple times.
Maybe this has changed in the age of social media influencers, maybe this is different for black people, but Japanese cops have always taken the discrete approach with me and the folks I’ve known (both Japanese and non-Japanese).