Bug in [[Special:LanguageStats]]
Fragment of a discussion from Support/LiquidThreads
Yes, it was still happening, as reported by malafaya, for the same problem as in the other thread (document.write or something, I believe) but in different circumstances. Purging always reproduced it.[1] The solution was "$wgResourceLoaderExperimentalAsyncLoading to false" (gerrit:62656).