Markdown "Open as Preview" shows a blank editor

Does this issue occur when all extensions are disabled?: Yes

- VS Code Version: 1.136.0
- OS Version: macOS 26.6.2

## Steps to Reproduce:

1. Open any markdown file

2. Run `Open as Preview` via button in the top-right, or command palette.

## Expected Behavior

The markdown file should be rendered as a preview in the editor.

## Actual Behavior

A blank editor is shown.

## Additional Information
- This issue is reproducible even with all extensions disabled and in a temporary profile.
- `Markdown: Open Preview` works correctly; only `Open as Preview` has this issue.