What I expected to happen was...:
I thought it would stop or maybe pass through the block
What actually happened was...:
It went through visually and could be picked up at the corners and edges
Steps to Reproduce:
1. make a water stream surrounded on all sides
2. put a slab (waterlogged if it doesn't work) at the top
3. put an item through the streams
If I'm understanding this correctly, the complaint is that items can be picked up through walls, which is WAI per MC-69762.