Fenced blocks marked with language are stripped out

Input code (as a markdown file referenced from documentation.yml):

(I added `\` at the start of ``` lines to get Github Issues to leave them alone).

```
To use without any build process:

\```html

\```

then

\```js
U.init(map)
\```

```

Output documentation:

```

To use without any build process:

then

With Webpack etc:

Expected: either just a regular code block, or ideally, with different syntax highlighting for js/html

Actual: those code blocks are completely missing from output.

Version 13.2.5

Running on CLI: `documentation build -f html -o docs --config documentation.yml --sort-order alpha src/index.js src/utilsGenerated.js`

https://github.com/stevage/map-gl-utils/blob/master/package.json#L16

For context: I want the language markers because the same README is also published to NPM and Github.