What I expected to happen was...:
The two walls that were connected would be short, still connected, and look normal.

What actually happened was...:
The sides of the walls that were on the opposite side of the connection between the two walls were invisible. Longer sides were still visible, as well as every other side except for the short, outside wall opposite of the connection between the two walls.

Steps to Reproduce:
1. Place 2 walls next to each other so that they form the connection.
2. Use the debug stick to set the "up" property to false on both walls
3. Observe

I was using this to act as a safety barrier for an airport parking garage that was 4 blocks wide, putting the walls in the middle so it looked even.

Tested on version 1.17.1 and the most recent snapshot as of Feb 20. Minor bug, simply just a missing texture.

Image w/ grass background - 23w07a

Other image - 1.17.1 with optifine installed. Optifine does not affect bug.