Why is Entity Framework (6) code-first inheritance strategy defaulting to TPT not TPH? - entity-framework

Question

According to everything I've read, the default EF code-first inheritance strategy is Table-Per-Hierarchy, but that is not what I'm getting - I'm getting TPT. (I'm on .NET 6.)
For the sake of this post, I've reproduced my issue with MS's own simplistic example:
public class Blog
{
public int BlogId { get; set; }
public string Url { get; set; }
}
public class RssBlog : Blog
{
public string RssUrl { get; set; }
}
public partial class MyContext : DbContext
{
...
public DbSet<Blog> Blogs { get; set; }
public DbSet<RssBlog> RssBlogs { get; set; }
...
}
But when I run add-migration Add_Blog_Tables I get the migration below...TPT not TPH!
Can anyone think of anything that might be causing my DbContext to default to TPH? And how would I tell EF that I want TPH when it is defaulting to TPT?
Thanks.
protected override void Up(MigrationBuilder migrationBuilder)
{
migrationBuilder.CreateTable(
name: "Blog",
columns: table => new
{
BlogId = table.Column<int>(type: "int", nullable: false)
.Annotation("SqlServer:Identity", "1, 1"),
Url = table.Column<string>(type: "nvarchar(max)", nullable: true)
},
constraints: table =>
{
table.PrimaryKey("PK_Blog", x => x.BlogId);
});
migrationBuilder.CreateTable(
name: "RssBlog",
columns: table => new
{
BlogId = table.Column<int>(type: "int", nullable: false),
RssUrl = table.Column<string>(type: "nvarchar(max)", nullable: true)
},
constraints: table =>
{
table.PrimaryKey("PK_RssBlog", x => x.BlogId);
table.ForeignKey(
name: "FK_RssBlog_Blog_BlogId",
column: x => x.BlogId,
principalTable: "Blog",
principalColumn: "BlogId");
});
}