HLOD proxy generation invalidates other clusters

When rebuilding a single HLOD proxy mesh in a world composition level that has many, the other proxy meshes will be marked as unbuilt and removed from the HLOD asset package.

• Open test project
• Open /Game/Developers/lmainguy/HLODBug/World
• Open the world composition tab & double click on the Sublevel0 to load that level
• Open the HLOD Outliner
• Click rebuild all
• At this point, all LODActor should be built
• Select one of the cube
• In the Details panel, check the "Overriden Light Map Res" property and change the value to 128
• At this point, the LODActor associated with this cube should be "Unbuilt"
• Right click on the "Unbuild" LODActor in the HLOD Outliner and select "Rebuild Proxy Mesh"

Result: The other LODActors are now "Unbuilt"

Expected: All LODActors should be built