I noticed that an automated truck we have running between a Quartz deposit and our main base runs empty on the way back to the home base, where it would normally get refuelled with coal.
When I was originally mapping out the autopilot, I could easily make the trip with a full stack (100) of coal. But whenever I leave the truck (the Unit) on its own, it runs dry about 20 meters before it can make the truck station at our main base.
However, when I manually fuel it up with a bit of coal so it can make the station, the truck station tops off the coal just fine. Then, when I follow the truck while it's on auto pilot, it reaches the quartz truck station fine, and on the trip back it has more than enough coal when it reaches the main base!
My theory: When the truck is out of the viewing/render range of a player, it bugs out causing it to use more fuel (coal, in my instance). This causes the truck to stop short of our main base, and us not getting any quartz.