Fences disconnect after swapping worlds

Short description of the issue: Some fences got disconnected after swapping worlds back and forth.

Steps to reproduce the issue: Load a survival world from the previous preview update and build a partial sugarcane farm with only pistons being placed, then swap to a creative mode world and build a sugarcane farm with a redstone contraption with pistons and observers, then swap back to the normal survival world.

Expected result: The fences stay attached together.

Actual result: Some fences got disconnected.