Marked 3.2.6: CommonMark (GFM) fails in normal document preview but works in Markdown Dingus

Marked 3.2.6 (1223), Setapp build — CommonMark (GFM) fails in normal preview
A minimal Markdown file does not render when the default processor is CommonMark (GFM). The preview instead displays essentially raw Markdown.
The same file renders correctly when the default processor is changed to MultiMarkdown.
CommonMark (GFM) also renders the same Markdown correctly in Help → Markdown Dingus.
Reproduced with files in both ~/Downloads and ~/Documents.
No Custom Rules are configured. Source View is disabled. The application was completely uninstalled and reinstalled, and the problem remains.
Debug logging shows:
readAndProcessMarkdown: cancelled before Conductor/processor
Reproduction:

1. Set Processor → CommonMark (GFM).
2. Open a .md containing # Test Heading and This should be bold.
3. Normal preview displays Markdown syntax rather than rendered HTML.
4. Change processor to MultiMarkdown → renders correctly.
5. Test CommonMark in Markdown Dingus → also renders correctly.