@JoeWintergreen, the “Build and Sync Lightmap UV” is simply a shortcut which will invoke UE4’s builtin lightmap generation with one additional step to actually save/sync generated UV into Staticmesh, so it’ll leave the “Lightmap Coordinate Index” as it’s. I should probably add a “Set Lightmap UV Index to” functionality. But back to your example, even it was set to 0 witch will most likely give you a lightmass warning regarding ovelapping UV but should not causing crash.
It would be great that you can send the affected mesh or even better the map so I can reproduce the issue.