It used to be (late 00's) that wikis had a big advantage in search results because backlinks were weighted heavily by the ranking algorithm and the dense connectivity of wikis meant that there were always a zillion backlinks to pages on a wiki.

Entirely possible that this is caught in a malfunctioning webspam filter. A wiki is largely indistinguishable in structure from an SEO link farm; indeed, any abandoned wiki can be and usually is turned into a link farm by bots. Maybe Google decided that its default was going to be to ban all wikis until a sufficient mass of evidence comes that it's trusted. Which, again, sucks for search quality, but the web and Google Search have both been pretty much dead for the last ~5 years anyway, we just rely on them anyway.