A little more info...
It definitely has something to do with the feedrate clashing with Mach3. I ran the same program a couple more times, and full speed, and every time it would cut those corners off, around line N92000. Then I ran it again, and when it got close to line N92000, I overrode the speed to 30% in Mach3, and it finished perfectly.
I have similar other pieces of this geographical topography, and they do the exact same thing too. At the very end of the cut it chops the corners off.
Can anybody help me understand what's happening?