BSPs will be built with light/shadow extrapolated even if they are placed in a hidden sub_level of the project at the time of building.
Also Tested in 4.10.4, 4.11 Preview 6 and Dev-Editor-CL-2877235
*Quick Repro - Open the Attached 4.12 Project and Build Lighting for SubLevel_01 only*
RESULTS: The BSP will pop back up and light will be built around it in Sub_Level_01 even though it is hidden via Sub_Level _02
EXPECTED: The BSP should not be built nor should lighting be built around it.
Head over to the existingAnswerHub thread and let us know what's up.