I'm currently working on 2 custom houses right next to riverwood. In game when I fast travel a certain distant away, whiterun in this case, upon fast traveling back to my custom marker, one of my houses will be invisible. I can fix the issue by entering and exiting the house, but that's not really a permanent solution. Also there are certain areas near the house that turns invisible that I CANNOT put Navmesh. I can't for the life of my figure out why it won't let me lay Navmesh there. I've tried so many different settings I could think of and even tried letting the game auto navmesh that area, it just skips over it. I did notice while doing this, that I had previously deleted some green lines along the navmesh that seem to make large grids across the landscape. Could that have something to do with it?