• • • • Not actually a bug, it was a feature I was not aware of.  Please close this, thanks! ****

When a hopper minecart moves off of a powered activator rail, it gets stuck in a locked state and no longer picks up items.  This happens when the minecarts moves off of the rail, or when the rail is removed from under the minecart. 

HOW TO REPRODUCE:

Place a rail-line with a powered activator rail along it.

Push a hopper minecart across the activator rail.

Throw items on the minecart.