[Bug][Libra 2] Side loaded simplified Chinese books content is garbled

[binshuohu]

The issue is as described in the title, in ver 4.31.19086. It has nothing to do with character encoding. I've tried fonts in both GBK and GB2312 character sets and experienced the same problem. Chinese characters in the content just don't show up on the page while chapter info in the table of content and header/footer is rendered correctly.
This issue only happens on side-loaded books in this version. For Chinese books bought in kobo store, both simplified and traditional Chinese can be rendered properly.