multi-line subtitle is rendered outside the `subtitle` class

[herosi]

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

When multi-line subtitle is used like the following code, the content is rendered outside the subtitle class like the following figure.

---
format:
  revealjs
title: |
  aaa\
  bbb
subtitle: |
  xxx\
  yyy
---

Steps to reproduce

No response

Actual behavior

When multi-line subtitle is used like the code above, the content is rendered outside the subtitle class like the figure above.

Expected behavior

The content should be inside the subtitle class.

Your environment

OS: Windows 11
IDE: VS Code

Quarto check output

quarto check
Quarto 1.8.26
[>] Checking environment information...
      Quarto cache location: C:\Users\xxxx\AppData\Local\quarto
[>] Checking versions of quarto binary dependencies...
      Pandoc version 3.6.3: OK
      Dart Sass version 1.87.0: OK
      Deno version 2.3.1: OK
      Typst version 0.13.0: OK
[>] Checking versions of quarto dependencies......OK
[>] Checking Quarto installation......OK
      Version: 1.8.26     
      Path: C:\Users\xxxx\AppData\Local\Programs\Quarto\bin
      CodePage: 932

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

(|) Checking LaTeX........(/) Checking LaTeX........(-) Checking LaTeX........(\) Checking LaTeX........
[>] Checking LaTeX....................OK
      Tex:  (not detected)

(|) Checking Chrome Headle(/) Checking Chrome Headle[>] Checking Chrome Headless....................OK  
      Using: Chrome found on system
      Path: C:\Program Files (x86)\Microsoft\Edge\Application\msedge.exe      
      Source: Windows Registry

(|) Checking basic markdow(/) Checking basic markdow(-) Checking basic markdow(\) Checking basic markdow(|) Checking basic markdow(/) Checking basic markdow(-) Checking basic markdow(\) Checking basic markdow(|) Checking basic markdow(/) Checking basic markdow(-) Checking basic markdow(\) Checking basic markdow(|) Checking basic markdow(/) Checking basic markdow(-) Checking basic markdow(\) Checking basic markdow(|) Checking basic markdow[>] Checking basic markdown render....OK

(|) Checking Python 3 inst(/) Checking Python 3 inst(-) Checking Python 3 inst(\) Checking Python 3 inst(|) Checking Python 3 inst(/) Checking Python 3 inst(-) Checking Python 3 in[>] Checking Python 3 installation....OK
      Version: 3.14.2
      Path: C:/Users/xxxx/AppData/Local/Python/pythoncore-3.14-64/python.exe
      Jupyter: 5.9.1
      Kernels: python3

(\) Checking Jupyter engine render....C:\Users\xxxx\AppData\Local\Programs\Quarto\share\jupyter\jupyter.py:28: DeprecationWarning: 'asyncio.WindowsSelectorEventLoopPolicy' is deprecated and slated for removal in Python 3.16      
  asyncio.set_event_loop_policy(asyncio.WindowsSelectorEventLoopPolicy())    
C:\Users\xxxx\AppData\Local\Programs\Quarto\share\jupyter\jupyter.py:28: DeprecationWarning: 'asyncio.set_event_loop_policy' is deprecated and slated for removal in Python 3.16
  asyncio.set_event_loop_policy(asyncio.WindowsSelectorEventLoopPolicy())    
[>] Checking Jupyter engine render....OK

[>] Checking R installation...........(None)

      Unable to locate an installed version of R.
      Install R from https://cloud.r-project.org/