Trucks drive to dumping spot and then refuse to dump

Screenshot shows two trucks full of waste, sitting at the edge of the land where waste dumping is allowed, showing the "no place to dump" icon. Dumping designation is controlled by tower that allows waste dumping. The only possible reason the trucks would even be at that location is because they went there to dump their loads... so why aren't they dumping?

I've also noticed idle trucks while ore sorters are full of dirt, when there are places to dump dirt (including this place).

My theory about both issues is that a truck reserves the spot where it's going to dump, and no other truck can be dispatched to dump at that spot. Since each truck spends much more time driving to the spot than actually dumping, this reservation slows the whole operation to a near standstill.

If you load the save, you'll see more trucks come to dump slag and dirt. Some of them do dump, others get stuck just like these two. When I added more designations and then removed them again, it all seemed to resolve -- everyone waiting was suddenly able to dump.

Generally this is due to, as you said, designation reservations. This isn't a bug, but the way to solve it is to a) have your designation/bridge be wider, so they can do 2+ at once b) have them instead transfer the waste to a nearby storage bin, which then goes to either a stacker or another storage bin which you assign 1-2 trucks to (or more if needed) which then outputs to a mining tower that includes the dumping area, but doesn't allow any others to import to it

I feel like a truck that's there at the spot should be able to take the reservation from one that's still on the way. I believe a similar change was recently made, where a truck that finishes its current job can take a nearby job that a more distant truck is currently on the way to.

I don't remember a change like that, but it would be interesting to have, but only if the truck is already waiting at the dumping zone. Further away, and it might cause problems. Though it still might cause problems, I just can't think of any right now, so long as it stays to the same mining tower area