Minecart unloading

If a minecart stands on a detector rail, it CAN'T be unloaded in to a hopper.
But if it stands on a detector rail it CAN be loaded by a hopper

What I expected to happen was...:
If a minecart stands on a detector rail it can be loaded and unloaded in/by a hopper

What actually happened was...:
Only can be loaded by a hopper but not unloaded (only one item)

Steps to Reproduce:
Only make it possible to load and onload on a detector rail from or in hoppers