SwiftUI : Why preferenceKey is not read/called - ios

Question

From the code below, I can't figure out why Preference key is not read from the view put in background of HStack. Strangely, If I put that BGViewSetter to the Text Views inside ForEach loop, it works. Unless I have ommitted something too obvious, it looks like a bug. Anybody can confirm it? Thanks
xcode12.2
ios 14.2
struct TestRectPreference : Equatable
{
var frame : CGRect
}
struct TestRectPreferenceKey : PreferenceKey
{
typealias Value = TestRectPreference
static var defaultValue: TestRectPreference = TestRectPreference(frame: CGRect.zero)
static func reduce(value: inout TestRectPreference, nextValue: () -> TestRectPreference)
{
value.frame = nextValue().frame
}
}
struct BGViewSetter: View {
var body: some View {
GeometryReader { geometry in
RoundedRectangle(cornerRadius: 25.0)
.fill(Color.init(#colorLiteral(red: 0.9372549057, green: 0.3490196168, blue: 0.1921568662, alpha: 1)))
.preference(key: TestRectPreferenceKey.self,
value: TestRectPreference(frame: geometry.frame(in: .global)))
}
}
}
struct FinalView : View
{
#State var offsetX : CGFloat = .zero
#State var info = ""
#State var size : CGFloat = .zero
var body: some View
{
VStack
{
Text("FinalView : \(info)")
HStack
{
ForEach( 1 ..< 10)
{ i in
Text("\(i)")
.frame(width: 100)
.opacity(0.8)
}
}
.background(BGViewSetter())
.animation(.easeOut)
.gesture(
DragGesture()
.onChanged
{ gesture in
self.offsetX = gesture.translation.width
}
.onEnded
{ _ in
self.offsetX = .zero
}
)
.offset(x: self.offsetX)
Text("Footer")
Divider()
Spacer()
}
.onPreferenceChange(TestRectPreferenceKey.self)
{
self.size = $0.frame.height
self.info = "Pref Chng : \(self.size)"
}
}
}
struct PreferenceKeyTest_Previews: PreviewProvider {
static var previews: some View {
FinalView()
}
}