Added built levels to a persistent level, every static mesh gone

So I had built 3 seperate levels to be combined in a persistent level. I’m using static mesh to build the geometry of the level. So I made a persistent level, added the three levels I had build, built the lighting and BOOM, every single static mesh got reset to nothing. And to the wrong type so I can’t just bulk edit it, I literally have to create the entire levels from scratch again.

Why did this happen? Weeks of work just gone in a flash. How can I prevent this from happening again? What am I doing wrong? Their transforms got reset as well so I can literally do nothing to change this except start over.