visionOS RealityKit's physics simulation stops for certain entity.scale values

The setup:


My Vision Pro app loads uszd models created by a 3rd party app. These models have to be scaled to the right dimension for RealityKit.
I want to use physics for realistic movement of these entities, but this does not work as expected.
I thus wrote a demo test app based on Apple's immersive space app template (code below).
This app has two entities, a board and above a box that is a child of the board.

Both have a collision component and a physics body.

The board, and thus also its child the box, are scaled.

The problem:


If the scale factor is greater or equal 0.91, the box falls under gravity towards the board where it is stopped after some movements. This looks realistic.

However, if the scale factor is below 0.91, even 0.9, the movement of the box is immediately stopped on the board without any physics simulation.

Unfortunately, I am not able to upload screen recordings, but if the demo app is executed, one sees the effect immediately.

The question:


I cannot imagine that this behavior is correct. Can somebody confirm that this is a bug? If so I will write a bug report.

My demo app uses Xcode Version 16.0 (16A242d), simulator Version 16.0 (1038) and visionOS 2.0.

The code:

import SwiftUI
import RealityKit
import RealityKitContent

struct ImmersiveView: View {
    @Environment(AppModel.self) var appModel
    
    var body: some View {
        RealityView { content in
            let boardEntity = makeBoard()
            content.add(boardEntity)
            
            let boxEntity = makeBox()
            boardEntity.addChild(boxEntity)

            moveBox(boxEntity, parentEntity: boardEntity)
        }
    }
    
    func makeBoard() -> ModelEntity {
        let mesh = MeshResource.generateBox(width: 1.0, height: 0.05, depth: 1.0)
        var material = UnlitMaterial(); material.color.tint = .red
        let boardEntity = ModelEntity(mesh: mesh, materials: [material])
        
        let scale: Float = 0.91 // Physics does not run if scale < 0.91
        boardEntity.scale = [scale, scale, scale]
        
        boardEntity.transform.translation = [0, 0, -3]
        
        boardEntity.generateCollisionShapes(recursive: false)
        
        boardEntity.physicsBody = PhysicsBodyComponent(massProperties: .default, 
                                                       material: PhysicsMaterialResource.generate(friction: .infinity, restitution: 0.8), 
                                                       mode: .static)

        return boardEntity
    }
    
    func makeBox() -> ModelEntity {
        let mesh = MeshResource.generateBox(size: 0.2)
        var material = UnlitMaterial(); material.color.tint = .green
        let boxEntity = ModelEntity(mesh: mesh, materials: [material])
        
        boxEntity.generateCollisionShapes(recursive: false)
        
        boxEntity.physicsBody = PhysicsBodyComponent(massProperties: .default, 
                                                     material: PhysicsMaterialResource.generate(friction: .infinity, restitution: 0.8), 
                                                     mode: .dynamic)

        return boxEntity
    }
    
    func moveBox(_ boxEntity:Entity, parentEntity: Entity)  {
        // Set position and orientation of the box
        let translation = SIMD3<Float>(0, 0.5, 0)
        // Turn the box by 45 degrees around the y axis
        let rotationY = simd_quatf(angle: Float(45.0 * .pi/180.0), axis: SIMD3(x: 0, y: 1, z: 0))
        let transform = Transform(rotation: rotationY, translation: translation)
        boxEntity.transform = transform
    }
    
}