Just an idea.
The last position that will be recorded will be the last data that that was successfully written to the hardware from the file buffer.
Now the power going out will be for all intents and purposes a random event, there is no guarantee that any particular data will be successfully written to hardware before a power outage.
The only way I can see everything happening gracefully is with a UPS, that once detects a mains outage is able to shutdown gracefully. But the machine itself may not be in the exact position you expect due to inertia and any other unknown factors mechanical or electrical.
So basically data not being written the disk is more of a limitation of an OS when power is removed suddenly.
Also one would hope the file being written to is not stored in tmpfs (ram) then written to disc during the shutdown process of Linuxcnc.
