Walls not properly connected when /fill used

When i use the /fill command with walls, one of them is not well connected

What I expected to happen was...:
When i used the "/fill" command with walls, they would connect properly

What actually happened was...:
No matter what size i try to make it, one of the walls is not connected as it should be, specifically, the one that's below the upper last and furthest from the initial xz cord. (Furthest xyz cords, basically.)

Steps to Reproduce:
1. Use "/fill ~ ~ ~ ~n ~n(>0) ~n minecraft:andesite_wall"

Example: /fill ~ ~ ~ ~1 ~1 ~1 minecraft:andesite_wall

2. Assumming ~ = 0, look for the cord 1 0 1.

P/S: It can happen with any size, just look for the one that's below the furthest wall