This one tripped me up until I saw the obvious.

Hexo posts have a tags attribute in their "frontmatter" (the meta-data at the top of a post).

Initally, I incorrectly tried putting a comma separated word list in that field.

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tags: hexo, markdown, solved

This led to Hexo generating a single tag called "hexo, markdown, solved", which wasn't what I wanted.

Scouring the documentation didn't seem to flaunt any obvious answers, and even the primary example pages that Google returns don't use tags that much.

But, I did stumble across one, and the light went on. Dummy, it's YAML. That means the "frontmatter" is an array.

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tags:
  - hexo
  - markdown
  - solved