I made a little island out of mountain pieces and put a "nordic dwelling" (one of the circular mounds with interior) on it. Looked fine in the editor, but in-game there was just the interior of the dwelling floating above the water. No exterior walls or island. On a hunch, I checked "Is Full LOD" on all the references, and there they were in the game. I looked at other places where the same statics were used, and the box was not checked. From the description, that box should only affect being seen from great distances, not from the same cell. Maybe being a hair's width from the "You cannot go that way" border had something to do with it?