QWebEngineDownloadItem::totalBytes() always returns -1

`QWebEngineDownloadItem::totalBytes()` always returns `-1` even when it is easy to determine the file size.  For example, the following link always returns -1.

https://download.stoutner.com/privacy-browser/Privacy%20Browser%203.10.1.apk

Or, any link from this page or any other page I have tested.

https://www.stoutner.com/privacy-browser-android/changelog/

It is easy to determine the file size from the example link using an HTTP HEAD command, which makes me think that there is something broken in the `totalBytes()` method.