Select + Distinct + Select Collection results in incorrect SQL and results

Start with the MyDbContext and DistinctBug.Run reproduction, comparing the generated SQL and ShapedQueryExpression for the two LINQ queries. Trace the Select, Distinct, and collection-navigation translation path, then verify that the second query materializes tbl2s and preserves regression coverage for the reported case.

public class MyDbContext : DbContext
{
    public DbSet<tbl1> tbl1s { get; set; }
    public DbSet<tbl2> tbl2s { get; set; }

    protected override void OnConfiguring(DbContextOptionsBuilder optionsBuilder)
        => optionsBuilder
        .LogTo(Console.Out.WriteLine)
        .UseNpgsql("...");
}

[Table("tbl1")]
public class tbl1
{
    public int id { get; set; }
    public string col1 { get; set; }

    public virtual List<tbl2> tbl2s { get; set; }
}

[Table("tbl2")]
public class tbl2
{
    public int id { get; set; }
    public string col1 { get; set; }
    public int tbl1id { get; set; }

    public virtual tbl1 tbl1 { get; set; }
}

public class DistinctBug
{
    public static void Run()
    {
        using (var db = new MyDbContext())
        {
            var ok = db.tbl1s.Distinct().Select(t1 => new { t1.tbl2s }).ToList();
            Debug.Assert(ok.All(item => item.tbl2s != null));
        }
        using (var db = new MyDbContext())
        {
            var buggy = db.tbl1s.Select(t1 => new { t1 }).Distinct().Select(t1 => new { t1.t1.tbl2s }).ToList();
            Debug.Assert(buggy.All(item => item.tbl2s != null));
        }
    }
}

The first query works as expected. SQL:

      SELECT t0.id, t1.id, t1.col1, t1.tbl1id
      FROM (
          SELECT DISTINCT t.id, t.col1
          FROM tbl1 AS t
      ) AS t0
      LEFT JOIN tbl2 AS t1 ON t0.id = t1.tbl1id
      ORDER BY t0.id, t1.id

In the second query, this SQL query is generated:

      SELECT t0.id, t0.col1
      FROM (
          SELECT DISTINCT t.id, t.col1
          FROM tbl1 AS t
      ) AS t0

The ShapedQueryExpression before materialization inject:

ShapedQueryExpression:
    QueryExpression:
        Projection Mapping:
        SELECT t0.id, t0.col1
        FROM Projection Mapping:
        (
            SELECT DISTINCT t.id, t.col1
            FROM tbl1 AS t
        ) AS t0
    ShaperExpression: new { tbl2s = EntityShaperExpression:
            EntityType: tbl1
            ValueBufferExpression:
                ProjectionBindingExpression: id:0,
                    col1:1,
            IsNullable: False
         == default(tbl1) ? null : EntityShaperExpression:
            EntityType: tbl1
            ValueBufferExpression:
                ProjectionBindingExpression: id:0,
                    col1:1,
            IsNullable: False
        .tbl2s }

There are no exceptions generated. The issue is that tbl2s on the result items are null, instead of a list of entities, as in the first query.

EF Core version: 5.0.2
Database provider: Npgsql.EntityFrameworkCore.PostgreSQL
Target framework: .NET 5.0
Operating system: Windows 10
IDE: Visual Studio 2019 16.8.5