falling objects (sand, gravel ext) onto fence both on land and underwater

What I expected to happen was...:
falling gravel to land on the fence post

What actually happened was...:
gravel became unbreakable unless fence removed and half a block above the fence

Steps to Reproduce:
1. place a fence post and allow any falling block to fall onto it
2. should look different compared to placed b;ocks on a fence
3. still there even if i relog