Further testing shows deletion does happen when I close Focus normally. The files are all there until I delete tab history or choose the last open ta, whereupon they are deleted. This did not happen, previously. Browser history would disappear, but not downloaded files.

If I instead go into app info (long press its laugh icon and select from the pop up menu), and then force stop the app, the downloaded files remain and are not deleted.

I have some choice words for the developers about this silent change in behavior.