You can use fc-list :lang=zh-cn
to list all fonts you install. I believe it is the bug in Ubuntu. Your system may use the wrong font to show simplified Chinese.
Take a look at your /etc/fonts/conf.d/64-language-selector-prefer.conf
file, you may find the following:
<alias>
<family>sans-serif</family>
<prefer>
<family>Noto Sans CJK JP</family>
<family>Noto Sans CJK SC</family>
<family>Noto Sans CJK TC</family>
</prefer>
</alias>
<alias>
<family>monospace</family>
<prefer>
<family>Noto Sans Mono CJK JP</family>
<family>Noto Sans Mono CJK SC</family>
<family>Noto Sans Mono CJK TC</family>
</prefer>
</alias>
What you need to do is move JP
item to last. Highly recommand you to install Google Noto.
You can found more here.