question

Property first or last are special cases of a subnode structure. While a first entered items stays on top of a tree node sub tree as long as it is available on this object, you have identified that it is not available once it has exited this object. So the first item won’t loose its references even if another items enters your object. The second item will become first, when previous item has left the object.

So a reference of first is not a suitable approach to track items on a large conveyor system. You need a smaller observation space around your robot station. You can build it for example by photo eyes. You recognize items entering your space and put them into an array. And once they have left your space you erase them from your array. Maybe you won’t need an array, perhaps it is sufficient to store an actual involved item in a label and reference this label in your logic structure.