What config do you have for mastodon.el in your init.el file?

Do you get any warnings or errors when starting Emacs?

If you have the name of the font you want to use in the LaTeX fragments, have you tried using a LaTeX header at the top of the file, e.g.

#+LaTeX_HEADER:\usepackage{mlmodern}

(Replace mlmodern with whatever typeface you prefer)

If you don’t want your .org file to become too cluttered at the start, you could alternatively use a setup file via #+SETUPFILE: /path/to/setup-file.org – create an org file e.g. setup-file.org, and include a line like

#+LaTeX_HEADER:\usepackage{mlmodern}

or whatever your fixed-pitch/monospace font is supposed to be, as well as all the other options you want to use for LaTeX (like other \usepackage declarations, \newcommand definitions etc.).

I see. What’s the latest version of webkit that doesn’t have this problem? Have you been able to downgrade to it to verify?