Since many versions, between every shulker box is a small gap.

I often saw this issue, but because I normally play with mods, I thought it was a Mod-Issue. Now I realized that this issue is indeed vanilla.

So, because between every shulker box is a tiny gap, the renderer produces small artifact-like pixels between the edges, like in this picture of my test setup:

As you can see, the edges have this weird pixels, which are the result of the small gap.

If you look close enough (like in my example), or use the Spyglass or Zooming, you can clearly see, that a small, disturbing (may only for 1080p/small resolution screens) gap is there:

This small gap exists on every axis, it's like the block having a padding: 1px; css built in.