Found that the Wikiapiary Footer Page Name ($wmgWikiapiaryFooterPageName) refuse to save on my wiki when I try to leave it blank.
https://houkai2nd.miraheze.org/wiki/Special:ManageWiki/settings#mw-section-styling
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https://github.com/miraheze/ManageWiki/pull/362 (and a follow-up with https://github.com/miraheze/ManageWiki/commit/4771142) should fix it. That will be deployed tomorrow alongside our 1.38 upgrade. Closing as resolved since while the issue is still present on-wiki, the issue is resolved within ManageWiki.