URL from SwiftUI file exporter treated as directory instead of file

I have a document-based SwiftUI app where the document is a file wrapper. The document has the following writable content types:

static var writableContentTypes: [UTType] { 
    [.bookDocumentPackage, .epub, .pdf] 
}

I am using .fileExporter to export the document to PDF.

.fileExporter(isPresented: $isPublishing,
    document: book,
    contentType: .pdf,
    defaultFilename: book.metadata.title) { result in

    switch result {
        case .success(let url):
            book.publishPDF(location: url)

        case .failure(let error):
            print(error.localizedDescription)
    }
}

The problem is the URL the file exporter returns is treated as a directory instead of a file.

I have the following code to create a PDF context to create the PDF:

func buildPDFContext(url: URL) -> CGContext? {
    // For now, just go with a standard 8.5 by 11 inch page.
    let pdfContext = CGContext(url as CFURL, 
        mediaBox: nil, nil)

    return pdfContext
}

When I call the function, I get the following error message in Xcode’s debug console:

CGDataConsumerCreateWithFilename: failed to open /URL/from/fileExporter for writing: Is a directory.

The PDF context is not created. Instead of a PDF file, I get my document file wrapper as a collection of files and folders.

I can work around this problem on Mac by using NSSavePanel instead of SwiftUI’s file exporter. But iOS has no equivalent to NSSavePanel.

I tried using the FileManager class to create an empty file at the URL and use that to create the PDF, but the URL is still treated as a directory. I also tried making PDF the only writable content type, but the error persists.

How do I get the URL the file exporter returns to be treated as a file instead of a directory?