As I have said before this is a long standing bug. Defrag Freespace doesn't work as intended, period. It will create more fragmentation (even if using the "do not allow fragmentation" option). That's one of the bugs that annoy me the most, because I don't use the normal defrag process because it can be utterly slow on big partitions. I usually defragment from the file list. But sometimes, you may need to fill up some gaps on your drive.