When a purpur pillar is placed with [axis=x], the north-facing side of the texture is oriented in a way that makes the shading look incorrect. (The same happens with [axis=y] and the west face.) The shading painted into minecraft's textures generally assumes that light is coming from above, so when the purpur pillar is placed next to a purpur block, it looks like light is coming from above on the purpur block and from below on the purpur pillar.

How to reproduce: place some sideways purpur pillars next to some purpur blocks and look at it from the north or west side to notice the difference.

Expected behavior: For the north and west side pillar textures to be rotated by 180 degrees.