Syntax highlighting: First character of line treated as comment

Reproduce the issue in VS Code with a qmd document and the shown keyboard shortcut for “My Quarto Preview”; compare it with “Tasks: Run Task.” Use “Developer: Inspect Editor Tokens and Scopes” to confirm the first character is classified as a comment, and consider the issue complete when starting the shortcut no longer causes that character to change color or persist after reopening the editor.

The first character of a line in a qmd document is (sometimes) gray instead of the color of the rest of the line. That happens with a plain paragraph, a heading (first "#"), a blockquote (">"), inside the YAML block etc.

From "Developer: Inspect Editor Tokens and Scopes", it appears that this first character is classified as a comment, and therefore has the comment color (gray in my syntax theme):

The gray color appears slightly later than the regular color, i.e. first coloring is correct, then first characters become gray, as if there is a second pass of syntax highlighting.

Strangely, this glitch is triggered when I execute a user task:

    {
      "label": "My Quarto Preview",
      "type": "shell",
      "command": "quarto-preview '${file}'",
      "isBackground": true,
      "presentation": {
        "reveal": "always",
        "panel": "new",
        "close": false
      },
      "runOptions": {
        "instanceLimit": 5,
        "instancePolicy": "terminateOldest"
      },
      "problemMatcher": []
    }

quarto-preview is a bash script which wraps quarto preview --no-browser. It implements custom port finding logic, and then copies the URL output by Quarto to the clipboard, to be pasted manually into "Simple Browser". It is a workaround for quarto preview having problems with file names or a path which contain spaces.

The script itself doesn't interact with VS Code in any way – except maybe quarto does, despite --no-browser?

Moreover, the problem is not triggered if the task is started through "Tasks: Run Task", only if started via a keyboard shortcut:

{
        "key": "ctrl+shift+k",
        "command": "workbench.action.tasks.runTask",
        "args": "My Quarto Preview",
        "when": "editorTextFocus && editorLangId == 'quarto'"
    }

The problem remains after the task is stopped, and even if the editor is closed and reopened. Closing and reopening the project in VS Code reverts it.