eleventyConfig.addExtension("md", { ... }) doesn't seem to be working for me

[pdaoust]

Using 11ty 3.0.0-alpha.1.

I'm trying to modify the markdown engine to squeeze a preprocessing step in between the default liquid preprocessor and the Markdown parser. (Specifically it uses mermaid-cli's Node.JS API to turn mermaid code blocks into inline SVG.) I'm using the pattern suggested by https://www.starfallprojects.co.uk/blog/markdoc-eleventy/#enabling-markdoc , in which I call eleventyConfig.addExtension("md", { compile: fn }), where fn is an async function that receives the content, etc and returns a transformed string.

11ty doesn't seem to be picking up my custom function. I can paste code if it's useful. But here's the gist:

let MermaidPreprocessingMarkdown = {
  compile: async function(str, inputPath, preTemplateEngine, bypassMarkdown) {
    console.log("trying to preprocess Mermaid");
    // stuff happens
    return preprocessedMarkdown;
  }
};

eleventyConfig.addExtension("md", MermaidPreprocessingMarkdown);

Can anyone see what's wrong with this? I don't see the logged message I put in as a smoke test.