Earlier quoted context omitted.
I've tracked down the root cause: I needed to disable two uBlock Filters in order to successfully load the page: Fanboy's Annoyance and Fanboy's Social. It seems that they block some JS related to social buttons ( https://japan-dev.com/assets/socialButton.9ddbb1cc.js ) and that makes the rest of JS responsible for the rest of the page to not execute (due to a failed request). Sorry for the false flag.
Oh wow thanks for checking, that's helpful to know. I should probably restructure things so that one component failing doesn't bring down the whole site if I can...
I also get a sha512 mismatch on the CF script.
(Degrading gracefully so content is readable even without client-side JS is an even better idea but that obv takes more effort to sort out on an existing SPA)
Looking forward to reading your article but my reverse-engineering stops here for now (: