What I expected:
That the item would go in the hopper even when powered, but would not leak out of the hopper until unpowered.
What happened:
The item would not go in the hopper since it was powered, rendering sorting with hoppers near impossible.
Comments

I think it makes more sense for the hopper to be able to collect items when powered, but not deposit them.

Feature requests should be posted on the MC forums here.
Thanks
This is the intended behaviour.