Tikz diagram does not render in HTML if PDF is also an output format unless `keep-tex` is set to `true`

Bug description

I am using the pandoc-ext/diagram extension to draw TikZ figures. My intended output formats are both HTML and PDF.

If I enable only one output format then there is no problem. But if I enable both (HTML and PDF) output formats then the TikZ figure does not render in the HTML output unless keep-tex is set to to true for the pdf format.

Steps to reproduce

Because the issue is related to files, specifically, the creation of a filename_files subdirectory in the output directory, I am including two files in the reproducible example below.

In a project directory, install the pandoc-ext/diagram extension:

quarto add pandoc-ext/diagram

Create a _quarto.yml with the following contents:

project:
  type: book
  output-dir: docs

book:
  title: "TikZ example"
  chapters:
    - index.qmd

filters:
  - diagram

format:
  html: {}
  pdf:
    #keep-tex: true
    pdf-engine: xelatex

Create a index.qmd with the following contents:

# Example

This renders TiKZ diagrams in markdown.

``` {.tikz}
%%| label: delta-graph
%%| filename: delta-graph.pdf
%%| alt: Diagram showing how the delta-graph relates to the other graphs.
\tikzset{cat object/.style=   {node distance=4em}}

\begin{tikzpicture}[]
\node [cat object] (Del)                {$D$};
\node [cat object] (L)   [below of=Del] {$X$};
\node [cat object] (I)   [right of=L]   {$I$};
\node [cat object] (F)   [left of=L]    {$F$};

\draw [->] (Del) to node [left,near end]{$\scriptstyle{d_X}$}     (L);
\draw [->] (I)   to node [below]        {$\scriptstyle{x}$}       (L);
\draw [->] (Del) to node [above left]   {$\scriptstyle{d_{F}}$} (F);

\draw [->,dashed] (Del) to node {/}(I);
\end{tikzpicture}

Render the output with the following command:

quarto render

Expected behavior

When opening index.html in a web browser it should render the TikZ image.

Actual behavior

The TikZ image is not rendered in the HTML unless keep-tex is set to true as a pdf format option.

Your environment

-OS: macOS 15.1.1

Quarto check output

Quarto 1.6.33
[✓] Checking environment information...
      Quarto cache location: /Users/asheikh/Library/Caches/quarto
[✓] Checking versions of quarto binary dependencies...
      Pandoc version 3.4.0: OK
      Dart Sass version 1.70.0: OK
      Deno version 1.46.3: OK
      Typst version 0.11.0: OK
[✓] Checking versions of quarto dependencies......OK
[✓] Checking Quarto installation......OK
      Version: 1.6.33
      Path: /Applications/quarto/bin

[✓] Checking tools....................OK
      TinyTeX: (not installed)
      Chromium: (not installed)

[✓] Checking LaTeX....................OK
      Using: Installation From Path
      Path: /Library/TeX/texbin
      Version: 2024

[✓] Checking basic markdown render....OK

[✓] Checking Python 3 installation....OK
      Version: 3.13.0
      Path: /Users/asheikh/dev/python3venv/bin/python3
      Jupyter: 5.7.2
      Kernels: python3

[✓] Checking Jupyter engine render....OK

[✓] Checking R installation...........OK
      Version: 4.4.2
      Path: /Library/Frameworks/R.framework/Resources
      LibPaths:
        - /Library/Frameworks/R.framework/Versions/4.4-x86_64/Resources/library
      knitr: 1.49
      rmarkdown: 2.29

[✓] Checking Knitr engine render......OK