We would need to see your upload code to help debug that issue.
This is dependent on what the server you are running on uses as the temp directory for uploads set in the php.ini file. Most servers using the Operating System /tmp folder. That folder is cleared out on reboot of the server (sometimes on a schedule as well), so they will not be persisted. If the host choose a different folder to store the uploaded files temporarily, then the host would have to create its own purge for that folder too, otherwise, the files would be kept.