There should have been visible, tangible cobblestone fences there.

Some Cobblestone fences are invisible with no hitboxes. Tried placing other blocks but couldn't.

Steps to Reproduce:
1. make a world in 1.7.10 and place cobblestone walls in a square. Say 25x25 to be sure.
2. Close world and game and reload game and world in 21w44a

you should find some of the walls are acting strangely