Some redirects to the localized versions do not work

I just spotted a tweet https://twitter.com/azu_re/status/1339219961269473281 that going to https://developer.mozilla.org/ja leads to redirect an error page, while https://developer.mozilla.org/ja/ (with trailing slash) works fine.
It reproduced on my machine. Could somebody take a look?

@myakura I can reproduce this too. Thanks for reporting this @myakura — I’ll pass it on to our of our devs to have a look at.

I’m tracking it here: https://github.com/mdn/content/issues/353
Haven’t tested every locale yet, but it’s happening in ja and fr but not in en-US, de, or sv-SE.