404.html not generated on root if defaultContentLanguageInSubdir true

My Hugo website is multilingual and use defaultContentLanguageInSubdir true

404 html pages are generated inside the /fr/, /en/, /nl/ folders.
No 404.html page is generated on the root folder /

And the redirection doesn’t work in case of wrong url.

BTW accessing directly /fr/404.html shows a nice 404 crafted page.

May be related to Multilingual site always get a noindex tag in root index.html ??

Visit this topic with solution: Global 404 page for a multilingual site