Based on Liu Xiaodan's story, it was so easy to be bullied in prison if one stood out too much. Since Liu Xiaodan lied about this case to those who met him, he was seen as some kind of weakling and was generally left alone for the span of his stay. The same happened in Cherry Medical, and he worked so hard to be so behaved in both places. He was so disconnected from the two places that he barely had an opinion of them.
Hung Guangjian had no copy of the release order from the judge or records from Wangguan. There was simply no way for him to know the truth—or realize that Liu Xiaodan learned how to lie through his teeth while in those places. The detective could only rely on his instincts to strive and yet fail without him knowing.
However, by the way, Liu Xiaodan described his days in both Wangguan and Cherry Medical; Hung Guangjian started to doubt that this man was the one they saw back at the court hearing.