Why can't I see my Warehouse from the inside?

[MartinJH]

Having got a little confused by Collision Meshes, I thought I’d go back and try to recreate the warehouse shape from scratch from memory. This is what I did:
Start Unreal engine
Games → Blank → Blueprint
Desktop
Maximum
Starter Content
Project Name → WarehouseWreckage

Create a ‘Created’ folder
Open Content Drawer
Right click on Content Folder → New Folder → Created

Create New Level
File → Newe Level → Basic → Create
File → Save Current Level As → navigate to Created folder → Main

Delete floor in Viewport

Create outside of Warehouse
Qunickly add → Place Actors Panel
Choose Geometry
Drag Box to Viewport
Rename Box Brush to WarehouseInside in details
Change Brush Settings values to
x → 3400
v → 2700
z → 752

Duplicate WarehouseInside to WarehouseOutside
Change Brush Settings values to 
    x -> 3460
    v -> 2760
    z -> 812

Save
Change WarehouseInside to Subtractive type.

Expected - to see box shape of warehouse.
Result - can see outside of Warehouse, but when inside Warehouse can see only sky.
What have I done wrong?

[Merengil]

It sounds more like a quick fix to me, but I don’t have the problem anymore when I duplicate the subtract box (so I have 2 of them instead of one). I’d be curious to hear the full answer though!

[MartinJH]

Thanks for your reply. I duplicated the subtract box and it indeed it works! I deleted the first one and it still works, so you only need one.
As you say, it’s a fix, not a cure.
Is this a bug in Unreal engine?