Anki defaults to CommonHTML for MathJax, which doesn't work when
using mathbf fonts (e.g. /usr/share/anki/web/mathjax/conf.js
and change
jax: ["input/TeX", "output/CommonHTML"],
to
jax: ["input/TeX", "output/HTML-CSS"],
.
This will be overridden whenever Ubuntu upgrades to the next version, so you will have to edit this again at that time (unless the bug in MathJax is fixed).
On older versions of Android, the default fonts for kanji use the Chinese
variants. To correct this, you can change the templates to have
<span lang="ja">...</span>
around the kanji parts.