I ran into this same issue. In the initial Xcode 12 developer Beta it looks like the default placement of tool bar items is at the bottom. To fix this I had to explicitly identify where I wanted the items to go.
Having said that, the Add button disappears when you tap on it (both live preview and simulator). Not sure why this is happening as I can see the code is executing on each body call).
.toolbar {
ToolbarItem(placement: .navigationBarLeading) {
Button("Add", action: makeSandwich)
}
#if os(iOS)
ToolbarItem(placement: .navigationBarTrailing) {
EditButton()
}
#endif