Simplesearch of modular pages not working

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I have simplesearch working on several production sites, but they use a standard page template. I am using modular pages on a new site and simplesearch is only searching content on the parent page. How do I search the child modules?

simplesearch.yaml

filters:
    category: []

frontmatter of modular.md

content:
    items: @self.modular
    order:
        by: default
        dir: asc
taxonomy:
    category:
        - page

I have tried empty frontmatter on the module item and I have tried adding, neither works.

taxonomy:
    category:
        - page