`background-image` not embedded when `embed-resources: true`

it appears that the background-image is never embedded in the html file for some reason.
If the background image is set with {background-image="assets/background.jpg"}, it is embedded.

The following setup in the yaml, will result in slides without background.
While, it's working fine when the generated html is at the same level as the qmd file it comes from.

format:
  revealjs:
    embed-resources: true
    background-image: background.jpg
    output-file: docs/index.html

The image path is not replaced by a the base64 version of the image.
From the HTML file generated:

      // Parallax background image
       parallaxBackgroundImage: 'background.jpg',

Example: https://github.com/mcanouil/quarto-issues/tree/main/quarto-cli%23980
Version: 0.9.449
Originally from: quarto-dev/quarto-cli#951

Start with the linked quarto-issues#980 reproduction and its YAML configuration, comparing output when index.html is nested under docs/ with output at the qmd level. Trace the revealjs handling of embed-resources and background-image, then verify that the generated parallaxBackgroundImage uses an embedded base64 image in both locations.