Put the summary of the bug you're having here

What I expected to happen was...:
I expected sand to return to "solid block" state when colliding with the top of fence blocks.

What actually happened was...:
The sand block never changed back from an entity. It stopped at what appeared to be the top of the fence's collision box. Some side-effects of this include periodic "hopping" of the sand entity, including no longer having a collision box that players can "impact".

Steps to Reproduce:
1. Place fence anywhere.
2. Place a sand block in a position to deliberately fall on top of the fence.
3. Observe the stuck sand over the fence.