piglens wont pick up gold unless they can stand on the block the gold is on

I tried to create a piglen bartering farm from scratch using what I know about their mechanics when I discovered that if a piglen cannot stand on the block that the gold is thrown on they will not pick it up. I am unsure if this is a bug or if that's how their behavior was intended but it made what I thought would be an easy build into a struggle to work around.