For the first 100 times the files is supposed to be suffixed by an index. For example, test (1).txt
For the 101 time (and after) it is supposed to be suffixed with a timestamp. For example, test - 2019-03-28T08:45:29.355Z.txt
Using colons (:) in filenames on Windows is not valid thus this results the 101st file to be downloaded to weird folders depending on the position and the numbers of the colons in the filename.
Chrome just omits the colons from the filenames on Windows: test - 2019-03-28T093845.393.txt WebEngine should adapt this behavior.
NOTE: this makes tst_QWebEngineDownloadItem::downloadUniqueFileNameWithTimeStamp auto test fail on Windows.