Table of contents title is inserted surprisingly early

I have:

• searched the issue tracker for similar issues
• installed the latest version of Quarto CLI
• formatted my issue following the Bug Reports guide

Bug description

I would expect a filter to be able to operate on nil metadata in general. This allows an extension to provide reasonable backups for that type. However, a subset of metadata get filled early on in the process, likely linked to metadata with translations.

In particular, this means that items like the toc-title can't have filter-based backups without opting into an "always override" approach (or coming up with special non-standard metadata names).

The label is bug, but I would characterize this somewhere in the surprising behavior spectrum, as it's likely unintended but after searching through the javascript, I can see why it might happen and be hard to make this metadata perform like others.

Steps to reproduce

---
title: "TOC Title Example"
filters:
  - toc-title.lua
toc: true
format: typst
---

## Heading

This filter should have renamed the TOC to "Filter-provided title" because toc-title is not provided.

where toc-title.lua is:

-- replace the title if not provided
function Meta(meta)
  if meta["toc-title"] == nil then
    meta["toc-title"] = pandoc.MetaString("Filter-provided title")
  end
  return meta
end

Actual behavior

The title is "Table of contents".

Expected behavior

The title would be "Filter-provided title".

Your environment

• IDE: Positron 2026.08.1 build 2
• OS: Windows 11 Home 25H2

Quarto check output

Quarto 1.10.16
[>] Checking environment information...
      Quarto cache location: C:\Users\chris\AppData\Local\quarto
[>] Checking versions of quarto binary dependencies...
      Pandoc version 3.8.3: OK
      Dart Sass version 1.87.0: OK
      Deno version 2.7.14: OK
      Typst version 0.14.2: OK
[>] Checking versions of quarto dependencies......OK
[>] Checking Quarto installation......OK
      Version: 1.10.16
      Path: C:\Users\chris\AppData\Local\Programs\Quarto\bin
      CodePage: 1252

[>] Checking tools....................OK
      TinyTeX: (external install)
      Chrome Headless Shell: (not installed)
      VeraPDF: (not installed)

[>] Checking LaTeX....................OK
      Using: TinyTex
      Path: C:\Users\chris\AppData\Roaming\TinyTeX\bin\windows\
      Version: 2026

[>] Checking Chrome Headless....................OK
      Using: Chrome found on system
      Path: C:\Program Files\Google\Chrome\Application\chrome.exe
      Source: Windows Registry

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

[>] Checking R installation...........OK
      Version: 4.6.1
      Path: C:/PROGRA~1/R/R-46~1.1
      LibPaths:
        - C:/Users/chris/AppData/Local/R/win-library/4.6
        - C:/Program Files/R/R-4.6.1/library
      knitr: 1.51
      rmarkdown: 2.31

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

[>] Checking Python 3 installation....(None)

      Unable to locate an installed version of Python 3.
      Install Python 3 from https://www.python.org/downloads/

[>] Checking Julia installation...

(fyi Julia is installed and on path, but the check doesn't emit anything. Seems irrelevant to this issue. Also replicates on 1.10.18.)

Start with the provided Quarto document and toc-title.lua reproducer, then inspect the JavaScript metadata and filter-processing entry points around the Meta filter. Done means a missing toc-title can be supplied by the filter and the rendered Typst table of contents uses "Filter-provided title" without breaking explicitly provided titles.