Proof title replaces "Proof" in PDF output

Bug description

We document for proofs that:

As with theorems you can optionally include a heading as the first element of the div (or a name attribute) to give the environment a caption for typesetting (this typically appears in parentheses after the environment title).

But that doesn't seem to be true for PDF output. I don't think this is a regression, since it also doesn't work in 1.3.450.

Steps to reproduce

---
title: "Proofs"
format: pdf
---

::: {.proof}

## Title

Proof with title

:::

::: {.proof}

Proof without title

:::

Expected behavior

Something like format: html, the output begins with i.e. Proof (Title)

Actual behavior

The title appears, but "Proof" doesn't

Your environment

• OS: Mac 14.0 Sonoma

Quarto check output

Quarto 99.9.9
[✓] Checking versions of quarto binary dependencies...
      Pandoc version 3.1.11: OK
      Dart Sass version 1.69.5: OK
      Deno version 1.37.2: OK
[✓] Checking versions of quarto dependencies......OK
[✓] Checking Quarto installation......OK
      Version: 99.9.9
      Path: /Users/wickhamc/Documents/posit/quarto-cli/package/dist/bin

[✓] Checking tools....................OK
      TinyTeX: v2023.09
      Chromium: (not installed)

[✓] Checking LaTeX....................OK
      Using: TinyTex
      Path: /Users/wickhamc/Library/TinyTeX/bin/universal-darwin
      Version: 2023

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

[✓] Checking Python 3 installation....OK
      Version: 3.12.0
      Path: /usr/local/bin/python3
      Jupyter: 5.5.0
      Kernels: ir, julia-1.9, python3

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

[✓] Checking R installation...........OK
      Version: 4.3.2
      Path: /Library/Frameworks/R.framework/Resources
      LibPaths:
        - /Library/Frameworks/R.framework/Versions/4.3-arm64/Resources/library
      knitr: 1.45
      rmarkdown: 2.21

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