Does anyone know why my BSP geometry suddenly vanishes after a build?
I’ve put in some pretty standard stairs, using a level sequencer to move them on trigger press from level bp and it suddenly goes missing?
Convert your BSP brush to a mesh, after that it should work -> I’m not sure if you can move BSP brushes during runtime as they always have to be “rebuild” when they are moved 
The BSP dosen’t always work to convert to static mesh, it sometimes leaves walls and other things out. its not reliable.
Always be prepared to have a heart attack using BSP.
IMO BSP is only to fast prototype and you do need do make them all SM in a later stage.
In blender or to draw something similar doesn’t cost too much time, simply replace BSP with same pivot and your life will be easier.
Thats why is one reason BSP will be replaced eventually with new Geometry tools that directly edits and or creates simpler static meshes. EPIC talked about it on GDC 2017.
BSP equals:
Thanks for all your replies guys, been both interesting and helpful!