Oh OK, discard everything I said for now and use the build HLODs in the Build menu,
Your map should be working as is though, unless you’ve built HLODs before and deleted some stuff since then.
HLODs are simpler meshes generated by the engine to replace your buildings for viewing from a distance.
Anything close to you is loaded and shown as is, anything far from you is unloaded and your seeing the HLOD of it instead.
You have cell size and loading range in World Settings to determine the distance from a player this changeover happens.
When you build HLODs it turns all the “included in HLOD” assets in your map into simple meshes for this.