Placing a banner or a sign on the side of a block with any type of wall below the banner causes the wall to have its up blockstate set to true. This is because all banners are included in the wall_post_override block tag, not just standing ones.

What I expected to happen was....

Placing a banner or a sign on the side of a block with a wall below it to keep the wall as it is

What actually happened was....

The wall gained a bump as if I placed the banner or the sign standing upright on top of the wall itself

Steps to reproduce:

1. Place any type of wall
2. Place a block diagonal to it
3. Place a banner or a sign on the side of the block, above the wall
   →  The wall raises up to connect to the banner or the sign