Hey ,
Sorry for the delayed response. I was looking at the project you sent again as I still am unable to reproduce the issue outside of the provided project. I actually found that creating a blueprint and adding a static mesh to it, set to the meshes that were falling through the floor, would stream in and not fall through. This led me to discover that by selecting the mesh in the world as well as the content browser, I could right click on the mesh in the world and choose “Replace Selected Actors with…”. This would replace the static mesh that was falling through the floor with a new instance of itself. The only setting that needed to be set after replacing was to simulate physics again, and after doing so the new instance would collide as expected. I can’t say for certain why this is occurring but it seems to be an issue with the meshes that are already present in the world rather than the Should Block on Load checkbox. Let me know if doing this replacement helps on your end.
Cheers