OK. I raised this issue 1 month ago, but it mysteriously resolved itself and I closed the support thread.
The problem is now back in a different form. Now all of my 4 comics language feeds are not working / showing a 404 error:
I have also noticed that my 'comics' posts are now showing up in the feed of the 'blog' section of my site.
Previously the blog feed was only showing posts not in the category 'comics_xx' (e.g. 'comics_en'). (All of the comics technically are blog posts, but the website was coded to treat 'comics_xx' posts differently).
FYI the language translations are handled by the plugin Polylang.
Thanks in advance for users who can help me diagnose why WordPress is no longer displaying these feeds properly. Happy to answer your questions.