When letting a HTTP server run on localhost (e.g. python3 -m http.server inside this directory) and opening http://localhost:8080/download.bin via the URL bar in simplebrowser, the file gets downloaded as expected.
When doing the same thing a second time, the URL bar displays whatever URL was opened before the download, and nothing happens.
When doing it a third time, the download works again, the fourth time it fails again, and so on.
Downloading a file from the internet instead seems to work fine (but maybe it's just a timing thing - I'm testing this in a train via 3G). Downloading another file name seems to work as well.
This worked fine on Qt 5.9, and also can be seen outside simplebrowser (I discovered it because of failing tests in my testsuite).