Working my way through Hacking with macOs, the first SpriteKit (BubbleTrouble) example. Why does the sprite lose all X velocity on first contact with the physicsBody, but retains the Y velocity?

Abbreviated code:

class GameScene: SKScene {
    override func didMove(to view: SKView) {
        physicsBody = SKPhysicsBody(edgeLoopFrom: frame)
        physicsWorld.gravity = CGVector.zero

        let b = SKSpriteNode(texture: bT[1])
        b.setScale(1)
        b.physicsBody = SKPhysicsBody(circleOfRadius: b.size.width / 2)
        b.physicsBody?.linearDamping = 0
        b.physicsBody?.angularDamping = 0
        b.physicsBody?.restitution = 1
        b.physicsBody?.friction = 0
        b.physicsBody?.velocity = CGVector(dx: 150.0, dy: 150.1)
        b.position = CGPoint(x: 400, y: 300)
    }
}