You are dealing with deleted files, that is why du does not register used space, but dfdoes.
Deleted files only disappear after the owner process is stopped; they remain in use while that does not happen.
So to find the culprit process, I recommend you doing:
sudo lsof -nP | grep '(deleted)' Then have to killfor killing the process.
sudo kill -9 $(lsof | grep deleted | cut -d " " -f4)