It grow normally with a block at right and left side above it ! It=Cactus

What I expected to happen was...:

It should not grow but breakdown

What actually happened was...:

When I put a block at right side and left side above the cactus the cactus grow normally but when the is a block it should not grow

Steps to Reproduce:
1. I made a cactus farm
2. I added a block on the right side and left side above the cactus
3. Cactus grow normally it should drop with a block above it at the right and left