I'm in the process of flattening and dirt-ifying the island. I have a large storage area set up for dirt, all chained together. The first one accepts input from any source, the last one is set to empty, but is on an export route to a mining tower whos sole purpose is to control the dirt-laying. Dirt dumping is allowed globally, and there are plenty of dump orders set.
What's happening is, I set a new area to fill with dirt, covered by the mining tower. Trucks will go pick up dirt and fill the area as expected, but when the fill order is complete, every truck that had picked up dirt simply stops. They seem locked to the origin of their job, and since that mining tower has no further orders, they just sit there unable to find a job for their dirt load, even though dirt is allowed to be dumped globally, and the input for the dirt storage is still available.