Placing string on walls does not raise the wall

This was an amazing build trick, it's gone in this snapshot.

What I expected to happen was:
When placing the string, the wall was to add a pillar.

What actually happened was:
Nothing. Absolutely nothing happened.

Steps to Reproduce:
1. Place a minimum of 3 walls connected to each other horizontally
2. Place string on top of any one of the walls in the middle
3. In any other version, this would raise a pillar.