SwiftUI alignment guide layout. Why is the rectangle drawn out of bounds of the stack view?

AlignmentGuideLayout.swift

import SwiftUI

struct ContentView: View {
  var body: some View {
    VStack {
      // Why is the rectangle drawn out of bounds of the stack view?
      Rectangle().fill(Color.green)
        .frame(width: 100, height: 40)
        .alignmentGuide(HorizontalAlignment.center) { _ in 0 }
    }
    .border(Color.red)
    .padding()
    // To prevent clipping
    .frame(width: 300, height: 200)
    .border(Color.blue)
  }
}

import PlaygroundSupport
let view = ContentView()
PlaygroundPage.current.setLiveView(view)

Result: