Stacked walls create pillars when they should not, depending on the order of placement

The Bug

Since 20w06a, stacked walls connect, leaving no gaps between them and also not creating a pillar.
However, a pillar is still sometimes created depending on the order of placement.

Steps to Reproduce

1. Place at least 3 walls in a row along the x-axis (so that you are facing the wall when looking north or south)
2. Stack walls on both pillars at the ends of the wall you created
   
3. Stack a wall in the center
   → The bottom center wall creates a pillar