There's a very similar question on StackOverflow - https://stackoverflow.com/q/44556771/4002530 with no answers. This does seem like a bug doesn't it. swift import GameplayKit let tree = GKOctree(boundingBox: GKBox(  boxMin: vector_float3(x: -10, y: -10, z: -10),  boxMax: vector_float3(x: 10, y: 10, z: 10) ), minimumCellSize: 0.1) tree.add(NSObject(), at: vector_float3(x: 0, y: 0, z: 0)) tree.elements(at: vector_float3(x: 0, y: 0, z: 0)).count // 1, fine tree.elements(in: GKBox(  boxMin: vector_float3(x: -1, y: -1, z: -1),  boxMax: vector_float3(x: 1, y: 1, z: 1) )).count // 0, ?? tree.elements(in: GKBox(  boxMin: vector_float3(x: 1, y: 1, z: 1),  boxMax: vector_float3(x: -1, y: -1, z: -1) )).count // 0, well I tried // The original defining bounding box tree.elements(in: GKBox(   boxMin: vector_float3(x: -10, y: -10, z: -10),   boxMax: vector_float3(x: 10, y: 10, z: 10) )).count // 0, ***

One thing to chekc is that your diffable items are Reference types (class), and not Value types (structs).