Unexpected behavior expanding a back-navigation that contains derived entities with non-optional complex type where all properties are nullable

[stefanoirace]

Bug description

Dealing with EF10, with reference to the issue #37162, I have a more complicated use case that is not behaving as expected even after applying the fix #37196.

The non-optional complex type with all properties set as nullable is declared on a derived class that is retrieved through a back-navigation:

public class Project
{
    public int Id { get; set; }
    public List<ProjectProperty> Properties { get; set; }
}

public class ProjectProperty
{
    public int Id { get; set; }
    public int ProjectId { get; set; }
    public Project Project { get; set; }
}

public class ProjectLifetime : ProjectProperty
{
    public Lifetime Lifetime { get; set; }
}

public class Lifetime
{
    public DateTime? Start { get; init; }
    public DateTime? End { get; init; }
}

ProjectProperty has a relationship to Project with both Forward and Back-Navigation properties.
ProjectLifetime extends ProjectProperty and declares a complex property configured as required.

In my scenario ProjectLifetime properties are declared as nullable and the database values are all NULL.

When I perform a query like the following, I unexpectedly get the Lifetime property of the ProjectLifetime class unexpectedly null.

var project = await context.Projects.Include(p => p.Properties).SingleAsync();
Console.WriteLine(((ProjectLifetime)project.Properties.Single()).Lifetime is null);

The same code works fine with EF8.
I suppose that this could be a corner case while accessing complex type through a back navigation declared by a derived class.

Your code

await using var context = new BugDbContext();
await context.Database.EnsureDeletedAsync();
await context.Database.EnsureCreatedAsync();

context.Projects.Add(new()
{
    Properties = new List<ProjectProperty>
    {
        new ProjectLifetime()
        {
            Lifetime = new Lifetime()
        }
    }
});
await context.SaveChangesAsync();

context.ChangeTracker.Clear();

var project = await context.Projects.Include(p => p.Properties).SingleAsync();
Console.WriteLine(((ProjectLifetime)project.Properties.Single()).Lifetime is null); // Prints true

public class BugDbContext : DbContext
{
    public DbSet<Project> Projects { get; set; }

    protected override void OnConfiguring(DbContextOptionsBuilder optionsBuilder)
        => optionsBuilder
            .UseSqlServer("Server=localhost;Database=test;User=SA;Password=Abcd5678;Connect Timeout=60;ConnectRetryCount=0;Encrypt=false")
            .LogTo(Console.WriteLine, LogLevel.Information)
            .EnableSensitiveDataLogging();

    protected override void OnModelCreating(ModelBuilder modelBuilder)
    {
        modelBuilder.Entity<ProjectProperty>().HasOne(p => p.Project).WithMany(p => p.Properties).HasForeignKey(p => p.ProjectId);
        modelBuilder.Entity<ProjectLifetime>().HasBaseType<ProjectProperty>().ComplexProperty(p => p.Lifetime).IsRequired(true);
    }
}

public class Project
{
    public int Id { get; set; }
    public List<ProjectProperty> Properties { get; set; }
}

public class ProjectProperty
{
    public int Id { get; set; }
    public int ProjectId { get; set; }
    public Project Project { get; set; }
}

public class ProjectLifetime : ProjectProperty
{
    public Lifetime Lifetime { get; set; }
}

public class Lifetime
{
    public DateTime? Start { get; init; }
    public DateTime? End { get; init; }
}

Stack traces

Verbose output

The SQL STATEMENT produced by the repro-scenario:
SELECT [p1].[Id], [p0].[Id], [p0].[Discriminator], [p0].[ProjectId], [p0].[Lifetime_End], [p0].[Lifetime_Start]
      FROM (
          SELECT TOP(2) [p].[Id]
          FROM [Projects] AS [p]
      ) AS [p1]
      LEFT JOIN [ProjectProperty] AS [p0] ON [p1].[Id] = [p0].[ProjectId]
      ORDER BY [p1].[Id]

EF Core version

10.0.0

Database provider

No response

Target framework

.NET 10.0

Operating system

No response

IDE

Visual Studio 2026 18.0.2