In this scenario (the scenario in the title), the sorting interface consumes 1 item and you can't actually see the item anywhere, so you may feel like it got deleted. You can get it back by breaking the sorting interface.

Sorting interfaces shouldn't really accept items if they have nowhere to put them, and probably should function much more like a filtered block extender (no inventory of its own) than the current version.

The stack will show up in the left slot and will stay there until an output is found.

I don't think that this issue is closed, there is no left slot in the sorting interface. (That I can see?)

Also, it would be nice if the sorting interface showed what inventory it was connected to, as seeing the red side is slightly hard once it's connected.

Oh pff, I read sorting importer, woops. I'll have a look at this.

Looking into this, looks like we'd need to simulate adding any items to the connected inventory, and if that fails, not allowing items to enter the sorting interface.

Looks fairly done to me, easy to see when stuffed now. Closing.