SwiftUI Toolbar Issue with iOS 15

Question

This code, which displays a toolbar button to toggle the "bookmarked" property of a model object, works as intended in an iPhone simulator running iOS 16.0:

class Item: ObservableObject {
    @Published var bookmarked = false
}

struct ContentView: View {
    @StateObject var item = Item()

    var body: some View {
        NavigationView {
            Text(self.item.bookmarked.description)
                .toolbar {
                    MyToolbar(item: self.item)
                } //.toolbar
        } //NavigationView
    } //body
} //ContentView

struct MyToolbar: ToolbarContent {
    @ObservedObject var item: Item

    var body: some ToolbarContent {
        ToolbarItem {
            Button {
                self.item.bookmarked.toggle()
            } label: {
                Image(systemName: "bookmark")
                    .symbolVariant(self.item.bookmarked ? .fill : .none)
                    .foregroundColor(.yellow)
            } //Button
        } //ToolbarItem
    } //body
} //MyToolbar

In an iOS 15.0 simulator, however, the bookmark image in the toolbar fails to change between filled and empty (even though the underlying model value toggles correctly).

Am I using the .toolbar modifier incorrectly for iOS 15.0 code?

Edit: Incidentally, if I use the ToolbarItem {...} code directly in ContentView under the .toolbar modifier, then it works. But I would like to keep this code in an external ToolbarContent struct, if possible.