It's possible that we could support the horrible enwiktionary hacks, now that we are also putting heading information into strip markers...

FWIW previously headings from the old parser also generated strip markers, which were inside the heading text itself. This reminded me that https://ru.wikipedia.org/wiki/Модуль:RedirectChecker (admittedly an even hackier module! tracks whether section redirects lead to nowhere and adds an error category / displays an error message) would probably also have some issues going forward since it might no longer be able to look up the heading itself in the retrieved code (though so far I haven't heard any complaints about it).

T430391 may stem from the same issue AFAICT

Happy to report that actually, the issue was that although #parsoidfragment was something that was displayed in the page output while I was trying to troubleshoot it, on the wikitext level they were strip markers, like UNIQ--h-20--QINU (and it seems like their removal basically resolves the issue): https://ru.wikipedia.org/wiki/Модуль:Вложенный_список#L-366
I wish it was possible to know what level the heading is based on that marker, though. But being able to test it via TemplateSandbox really helped here anyway, thank you.