This might be related to what I posted in the steam feedback section earlier, depending on if I'm reading this correctly.
Per current play-through, though, I haven't seen behavior consistent with "wait until buffer empty before delivery" especially with trucks, as I definitely saw multiple deliveries with non-empty buffers. That said, when migrating from 60u trucks to 180u dump trucks, it certainly can appear they wait until the buffer is empty if your mining building is sufficiently far from the sorter due to the fact that deliveries are done serially (ie. building is reserved by a single vehicle, regardless of distance/drive time, and everyone else queues at the mining building until that vehicle delivers)
The serial behavior presents less of a problem with 60u trucks due to travel speed and the ability to structure storage inputs around single-load rejection, but 180u dump trucks are rarely single-load, making 'reject single-load' of questionable utility.
180u dump truck speed exposes the shortcomings of serial delivery behavior over remarkably short distances. Case in point: 1 mega excavator with 10 60u trucks was able to feed 6 T2 blast furnaces in my current play-through, but swapping to 5 180u dump trucks resulted in all trucks but one parked at the mining building waiting their turn to deliver to a sorter reserved solely for their use that sat inactive/idle 90% of the time.
Play-through context: Armageddon map, southern start point. Sorter facility located on cleared ground just south of crater rim copper deposit. Referenced mining operation is at the starting iron deposit, building WSW of the iron deposit. Delivery distance in that case is not terribly far.