Sheep does an animation of eating grass block, but the grass block doesn't change into dirt block. Sheep gets wool.

I have did an automatic wool farm. It relies on detector, which detect if grass block changes into dirt block, and then activating dispenser with shears. Then the wool is transported with a minecart with hopper in a grass block under sheep (grass block moved by piston)

What I expected to happen was...:
Grass block changes into dirt block and activates detector.

What actually happened was...:
Grass block doesn't change into dirt block. Stay as grass block.