Requesting completions from Pylance language server using request forwarding in VS Code extension

Question

Does Pylance provide completions for virtual documents? Should I be able to get completions from it using the vscode.executeCompletionItemProvider command?

I am making a VS Code extension that requires completions for Python code. I am following the Embedded Programming Languages guide and experimenting with code from LSP Example for Embedded Language using Request Forwarding.

My modified version of the example code works when I have Pyright enabled (and Pylance disabled) in my VS Code Extension Development Host. I get the expected completions. But when I have Pylance enabled (and Pyright disabled) I do not get Python completions. In both cases I get completions when editing a normal Python file.

Why wouldn't I be getting completions from Pylance through my extension?

My language client creates a virtual document and executes the command:

const clientOptions: LanguageClientOptions = {
    documentSelector: [{ scheme: 'file', language: 'html1' }],
    middleware: {
        provideCompletionItem: async (document, position, context, token, next) => {
            const originalUri = document.uri.toString(true);
            virtualDocumentContents.set(originalUri, document.getText());

            const vdocUriString = `embedded-content://py/${encodeURIComponent(originalUri)}.py`;
            const vdocUri = Uri.parse(vdocUriString);
            return await commands.executeCommand<CompletionList>(
                'vscode.executeCompletionItemProvider',
                vdocUri,
                position,
                context.triggerCharacter
            );
        }
    }
};

And I have a content provider for the embedded-content protocol:

workspace.registerTextDocumentContentProvider('embedded-content', {
    provideTextDocumentContent: uri => {
        const originalUri = uri.path.slice(1).slice(0, -3);
        const decodedUri = decodeURIComponent(originalUri);
        return virtualDocumentContents.get(decodedUri);
    }
});