Detaching an entity results in modify references

[AR1ES]

I disable AutoDetectChanges and load some related records from different tables. When I detach some entity, all references to it are automatically nullified.
Everything worked well on EF Core 2.*.

Steps to reproduce

public class Class1 {
	public long Id { get; set; }
	public long? Class2Id { get; set; }
	public Class2 Class2 { get; set; }
}
	
public class Class2 {
	public long Id { get; set; }
}
...
public void TestMethod() {
	using(var db = new MyContext()) {
		db.ChangeTracker.AutoDetectChangesEnabled = false;
		var t1 = db.Class1.Include(m => m.Class2).FirstOrDefault();
		
		...
		// db.Entry(t1).State - Unchanged
		
		// detaching sub entity
		db.Entry(t1.Class2).State = EntityState.Detached;
		
		// result: 
		// db.Entry(t1).State - Modified
		// db.Entry(t1).Reference(m => m.Class2) - IsModified = true
		// db.Entry(t1).Property(m => m.Class2Id) - IsModified = true
	}
}

Further technical details

EF Core version: 3.1.0
Database provider: Microsoft.EntityFrameworkCore.SqlServer
Target framework: NET Core 3.1
Operating system: Windows 10
IDE: Visual Studio 2019 16.4

[ajcvickers]

Note for triage: It looks like detaching is marking an FK as modified when it should not. Full repro:

public class Class1 
{
    public long Id { get; set; }
    public long? Class2Id { get; set; }
    public Class2 Class2 { get; set; }
}
	
public class Class2 
{
    public long Id { get; set; }
}

public class BloggingContext : DbContext
{
    protected override void OnConfiguring(DbContextOptionsBuilder optionsBuilder)
    {
        optionsBuilder
            .UseSqlServer(@"Server=(localdb)\mssqllocaldb;Database=Test;ConnectRetryCount=0");
    }
    
    protected override void OnModelCreating(ModelBuilder modelBuilder)
    {
        modelBuilder.Entity<Class1>();
        modelBuilder.Entity<Class2>();
    }
}

public class Program
{
    public static async Task Main()
    { 
        using (var context = new BloggingContext())
        {
            context.Database.EnsureDeleted();
            context.Database.EnsureCreated();

            context.Add(new Class1
            {
                Class2 = new Class2()
            });
            
            await context.SaveChangesAsync();
        }

        using (var context = new BloggingContext())
        {
                context.ChangeTracker.AutoDetectChangesEnabled = false;
                var t1 = context.Set<Class1>().Include(m => m.Class2).FirstOrDefault();
                var t2 = t1.Class2;
		
                Console.WriteLine("T1>" + context.Entry(t1).State);
                Console.WriteLine("T2>" + context.Entry(t2).State);

                context.Entry(t2).State = EntityState.Detached;

                Console.WriteLine("T1>" + context.Entry(t1).State);
                Console.WriteLine("T2>" + context.Entry(t2).State);
        }
    }
}