Nate,
It’s amazing that you have solved the real issue that I have suffered such a long time.
Mostly, this bug has been make my project 64 times more of draw calls because I had such a no choice for not using ISM because it’s modulized in very small chunks with sensitive shader.
So for fixing gap was just importing with each 90 degree of base static meshes, which costs 8 more each pre-rotated meshes consumes.
Thank you very much and I wish this should be known to other devs. Such a good news.