Alan, not sure if this will solve the riddle, but it has impacted us in the past where there have been bugs in this space.
Orphaned uploads are deleted periodically. As part of an upload, you should end up with an entry in the table uploads. If for any reason that is missing, the upload will be removed eventually.
Might be a line of enquiry.
to check the Upload model you can go into the rails console from Discourse directory and for example:
./launcher enter app
rails c
Upload.last(3)
Cheers