Some of my RSS feeds have stopped working / merged

To clarify the situation:

The feed should only display ‘comics’ posts. i.e. those that appear on the ‘Comics’ page for each language.

So for those currently viewable through the blog feed, the only ones that should be appearing through the ‘comics_en’ feed should be:

26/06/12 22:00 Thin Air
and
22/05/12 17:00 Type III

the others are blog posts, and should only appear in the blog feed.

So for some reason, the website is recognising that the ‘comics_en’ posts should appear on the Comics page, but not in the Comics feed.

what is ‘comics_en’ ? a category ?

Yes, ‘comics_en’ is the slug for the category ‘Comics’ (English language).

comics_el is the slug for ‘Κ?μικ?’ (Greek language)
comics_pt is the slug for ‘Quadrinhos’ (Brazilian Portuguese language)
comics_zh is the slug for ‘漫画’ (Chinese language)

…strangely the feeds now seem to be working… See:
https://www.stuartmcmillen.com/comics_en/feed/
https://www.stuartmcmillen.com/comics_el/feed/
https://www.stuartmcmillen.com/comics_pt/feed/
https://www.stuartmcmillen.com/comics_zh/feed/

Any idea why WordPress would be dropping in and out like this?

No. Really no idea.