macOS High Sierra 10.13.2 - Missing Disk space
I had a similar problem. I ran 'dd' to fill up the SSD and then deleted the files afterwards:

dd if=/dev/zero of=/tmp/zero.1.out bs=1048576 count=10240

I created a few of those (10GB each and some larger - 30 GB) until the SSD was nearly 100% full.

I then deleted the files with: rm /tmp/zero.*.out

Finally: df -h /
now reports the correct allocation with one oddity:
ifree read 9223372036854041010 which seems incorrect.

Disk Utility (First Aid) was not picking up any issue although the reported available space was clearly off by 50 GB (almost 50% of the entire capacity of my SSD partition).

Hope this helps.