When a user uploads files, as they upload them, they have to be owned by them.
What seems more likely is that the owner of the files is not being displayed correctly. That number you are seeing is the underlying "user id" that actually owns the files. On Unix/Linux, files are actually owned by user id rather than by the user name. Usually when the owner is displayed it's converted to text form, but if the conversion breaks the system will just display the number.
So, the question is, why are the usernames being displayed as numbers? And for that one, I'm not sure (unless that's what FTP always does/did).