APFS/High Sierra - `tmutil deletelocalsnapshots` not deleting snapshots
I think the least-recent time machine snapshots was corrupted - potentially from some system crashes caused by APFS bugs.

I tried running DIsk Utility's first aid, but that froze my computer for over 3 hours (maybe it ran out of space to work with?) - I ended up having to force-restart.

I solved the problem by running first aid booted from an external hard drive install of High Sierra. It removed some old snapshots. I then was able to boot from my normal drive and run tmutil deletelocalsnapshots... which successfully deleted the snapshots and freed up space