I’v been having this issue with Instance static meshes for a while now and it still hasn’t been resolved.
Meshes that snap together perfectly by drag and dropping in to the world, as well adding them using static mesh component.
But when you add these meshes using an instance static mesh component with a rotation applied the meshes will not line up with a small gap between each of the instances.
From what i can see its been an issue since 4.8 and is still present.
Here is a link to another person that had the issue: