Sorry, I've been really ill for several days. It's not the packaging system, it's the "update-notifier" that does the downloading. It appears that it used to be the package's responsibility to clean up the downloads and now it's the responsibility of the update-notifier. What they _should_ do is make it backward-compatible and say "delete this file if it exists, if it does not exist then proceed as normal".
Sorry, I've been really ill for several days. It's not the packaging system, it's the "update-notifier" that does the downloading. It appears that it used to be the package's responsibility to clean up the downloads and now it's the responsibility of the update-notifier. What they _should_ do is make it backward-compatible and say "delete this file if it exists, if it does not exist then proceed as normal".