So, from my rudimentary testing, here's what's happening.
- Coal hits 800, refuses to accept anymore.
- Conveyor extracts, causing it to fall below 800, even if it is by 1, so it starts accepting more trucks.
- Those trucks dump in, continuing to increase the unsorted input
- It eventually backs up
There is no way this is an intended feature, as what is the point of the larger ore sorter if that is the case?

A simple fix for this would be to have a threshold that can be set for accepting inputs, Say, only accept trucks if below 600, with 800 remaining the max, much like the storage containers.
What makes me believe this is in fact unintentional, is that the icon that the sorter "Is not accepting anymore of this product" Remains visible, even as more trucks are dispatched to it
Furthermore, From my memory, playing with ore sorting in update 2, this was not an issue.
