Can't add explicit value when adding for a property with AfterSaveBehavior=Save

[roji]

When configuring a property as ValueGeneratedOnAddOrUpdate and setting its AfterSaveBehavior to Save, the expectation is to be able to provide an explicit value. This works when updating, but not when adding.

Repro

await using var ctx = new BlogContext();
await ctx.Database.EnsureDeletedAsync();
await ctx.Database.EnsureCreatedAsync();

ctx.Blogs.Add(new Blog { Url = "foo", LastUpdated = new(2020, 1, 1) });
await ctx.SaveChangesAsync();

ctx.ChangeTracker.Clear();
var blog = ctx.Blogs.Single();
Console.WriteLine("LastUpdated after adding: " + blog.LastUpdated); // Null, value was ignored

blog.LastUpdated = new(1990, 1, 1);
await ctx.SaveChangesAsync();

ctx.ChangeTracker.Clear();
blog = ctx.Blogs.Single();
Console.WriteLine("LastUpdated after updating: " + blog.LastUpdated); // 1990-01-01, value was saved

public class BlogContext : DbContext
{
    public DbSet<Blog> Blogs { get; set; }

    static ILoggerFactory ContextLoggerFactory
        => LoggerFactory.Create(b => b.AddConsole().AddFilter("", LogLevel.Information));

    protected override void OnConfiguring(DbContextOptionsBuilder optionsBuilder)
        => optionsBuilder
            .UseSqlServer(@"...")
            .EnableSensitiveDataLogging()
            .UseLoggerFactory(ContextLoggerFactory);

    protected override void OnModelCreating(ModelBuilder modelBuilder)
    {
        modelBuilder.Entity<Blog>().Property(b => b.LastUpdated)
            .ValueGeneratedOnAddOrUpdate()
            .Metadata.SetAfterSaveBehavior(PropertySaveBehavior.Save);
    }
}

public class Blog
{
    public int BlogId { get; set; }
    public string Url { get; set; }
    public DateTime? LastUpdated { get; set; }
}

[ajcvickers]

The default before-save behavior for ValueGeneratedOnAddOrUpdate properties is Ignore. The model in this repro does not change this:

Model: 
  EntityType: Blog
    Properties: 
      BlogId (int) Required PK AfterSave:Throw ValueGenerated.OnAdd
      LastUpdated (Nullable<DateTime>) BeforeSave:Ignore ValueGenerated.OnAddOrUpdate
      Url (string)
    Keys: 
      BlogId PK

This is because SQL Server at least does not allow computed column values to be inserted, and we didn't want the default experience on SQL Server with computed properties to throw.

If I set the before-save behavior: property.SetBeforeSaveBehavior(PropertySaveBehavior.Save);, then I see the value being inserted as expected.