pencil kit issue--drawing can't stay on canvas

I want to use pencil kit in my swiftui app. I encapsulate PKCanvasView in UIViewRepresentable. I can draw on canvas with only one stroke. However, the image will disappear immediately when I release the finger.

Am I missing anything?

import SwiftUI
import PencilKit

struct ContentView: View {
    var body: some View {
        CanvasView()
    }
}

struct CanvasView: UIViewRepresentable {
    func makeUIView(context: Context) -> PKCanvasView {
        let canvas = PKCanvasView()
        canvas.tool = PKInkingTool(.pen, color: .green, width: 10)
        canvas.drawingPolicy = .anyInput
        return canvas
    }
    
    func updateUIView(_ uiView: PKCanvasView, context: Context) {
        
    }
}