In the 17w18b snapshot, I seem to be able to place snow layers on barriers. I cannot replicate this issue in 1.11.2, so it's a snapshot thing that can lead to barriers no longer being completely invisible. Aside from being able to place the snow manually, it will also form automatically in snowy environments.
Steps to replicate:
- Open the game in the 17w18b snapshot and enter or create a world with access to cheats.
- Get a barrier block and place it down.
- Place a snow layer on top of the barrier.