monaco-typescript not picking up configured modules from addExtraLib

monaco-editor version: 0.10.1
Browser: Chrome and Edge tried, presumably any
OS: Windows 10 tried, presumably any
Steps or JS usage snippet reproducing the issue:

1. Into the Monaco playground, type the following:

const externalDTS = `export declare function next() : string`;
const externalDTSFilename = 'external.d.ts';
const useExtraLib = true;

if (useExtraLib) {
    monaco.languages.typescript.typescriptDefaults.addExtraLib(externalDTS, externalDTSFilename);
}

var jsCode = `import * as x from "external";\nconst tt : string = x.next();`;

if (!useExtraLib) {
    var extModel = monaco.editor.createModel(externalDTS, "typescript",monaco.Uri.parse("file:///" + externalDTSFilename));
}

monaco.editor.create(document.getElementById("container"), {
    model: monaco.editor.createModel(jsCode,"typescript",monaco.Uri.parse("file:///main.tsx")), 
});

2. Run it.
3. Note that "external" is marked as an error - the module is not found.
4. Flip useExtraLib to false to seed the editor with the type definition file in another file/model instead of via addExtraLib.
5. Run it.
6. Note that "external" now resolves correctly.

(This example is adapted from a Stack Overflow answer, which in its entirety stopped working at some point.)

Reproduce the issue in the Monaco playground using the provided addExtraLib and createModel snippets, then compare module resolution with and without the extra library. The work is done when the external import resolves through addExtraLib as it does when the declaration file is seeded as another model.