iOS16.5.1の人はiPhoneで、以下をコピペするとわかります。
下のバーがなぜか起動時に本来の位置より上にあり、すぐに元の位置に戻ります。

この現象は
ToolbarItem(placement: .bottomBar) {
ToolbarItemGroup(placement: .bottomBar) {

どちらでも起こります。

早く治してクレメンス

import SwiftUI
import CoreData

struct ContentView: View {

    @State var selected:Int? = 1
    @State var selected2:Int?
    var body: some View {
        NavigationSplitView {

            List(selection: $selected){
                NavigationLink(value: 1){
                    Text("A")
                }
                NavigationLink(value: 2){
                    Text("B")
                }
                NavigationLink(value: 3){
                    Text("C")
                }
            }
        } content: {
            if let selected = selected {
               Text(String(selected))

                List(selection: $selected2){
                    
                    
                    ForEach(0..<10000){index in
                        
                        NavigationLink(value: index){
                            Text(index.description)
                        }
                    }

                }
                
                .toolbar {
                    ToolbarItemGroup(placement: .bottomBar) {
                        Text("A")
                    }
                    
                }
            } else {
                Text("No Selected")
            }
        } detail: {
            Text("Hi")
            if let selected2 = selected2 {
                Text(String(selected2))
            }else{
                Text("No Selected")
            }

        }

    }


}

なんか色々やってもダメ



import SwiftUI
import CoreData

struct ContentView: View {
    
    @State var selected:Int? = 1
    @State var selected2:Int?
    var body: some View {
        NavigationSplitView {
            
            List(selection: $selected){
                NavigationLink(value: 1){
                    Text("A")
                }
                NavigationLink(value: 2){
                    Text("B")
                }
                NavigationLink(value: 3){
                    Text("C")
                }
            }
        } content: {
            VStack{
                
                
                if let selected = selected {
                    Text(String(selected))
                    
                    VStack(alignment:.leading){
                        
                        ScrollView{
                            
                            
                            ForEach(0..<10000){index in
                                
                                Text(index.description)
                                
                            }
                        }
                        
                    }
                    
                } else {
                    Text("No Selected")
                }
                
            }
            .toolbar {
                ToolbarItem(placement: .bottomBar) {
                    Text("A")
                }
                
            }
        } detail: {
            Text("Hi")
            if let selected2 = selected2 {
                Text(String(selected2))
            }else{
                Text("No Selected")
            }
            
        }
        
    }
    
    
}