Item passed through slab visually but not actually

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