Copying non-syntax-highlighted plain text removes formatting

monaco-editor version: 0.21.2 (whatever is in the playground)
Browser: Firefox 81.0.2
OS: Mac OS X 10.14.6
Playground code that reproduces the issue:

// The Monaco Editor can be easily created, given an
// empty container and an options literal.
// Two members of the literal are "value" and "language".
// The editor takes the full size of its container.

monaco.editor.create(document.getElementById("container"), {
	value: "function hello() {\n\talert('Hello world!');\n}",
	language: "plain/text"
});

This is the playground "Hello world!" with language changed to plain/text.

Appearance of the text in the editor in the browser is plain text style:

Copy and paste the text into a rich text editor e.g. Microsoft Outlook's New Mail window, and it gets copied as text only, with no formatting:

Switch the language to something with syntax highlighting, like the original playground example of "javascript" and the result is:

Text in the editor in the browser:

Copying and pasting now does include the formatting:

I think there's something specific to the Monaco editor that is generating this effect, because outside the editor, if I copy a plain text file opened in the browser, and paste that, there is formatting: it appears as something like Courier New when pasted into the rich text editor, even though that isn't the default font.

Expected behaviour: copying includes the formatting regardless of whether there is syntax highlighting or not.
Actual behaviour: copying only includes the formatting if syntax highlighting is working.

Start with the Monaco Editor playground reproduction, comparing copy and paste behavior for plain/text and javascript in Firefox. Investigate the editor's clipboard behavior from that entry point; done means copying plain text preserves formatting when pasted into a rich text editor, as syntax-highlighted text does.