Items flowing in water streams get stuck on hopper collision mask

I noticed my storage system wasn't working, so I jumped into creative to fly around and debug.
I seen this, and at first I thought it was a ghost item, so I jumped in and I could pick it up.

The items on the hopper cannot be picked up by the hopper (Its an item sorter, this specific one sorts out cobble stone, but has granite stuck on top of it.), but they get stuck on its collision mask, instead of flowing over it. (As it worked in 1.8)

This really shouldn't happen, or else it would break almost any non-trivial water transport-->hopper sorter system.