Below is the original README

Only the 1st Owned Entity property can be configured by Fluent API

Steps to reproduce

A complete project is here: https://github.com/ganaware/TestEFCoreOwned

I defined an entity Book that has three Owned Entity properties: EnglishInfo, JapaneseInfo and ChineseInfo. The type of these properties is Info configured by fluend API.

    public class Book {
        public int BookId { get; set; }
        public Info EnglishInfo { get; set; }
        public Info JapaneseInfo { get; set; }
        public Info ChineseInfo { get; set; }

        public static void OnModelCreating(ModelBuilder modelBuilder) {
            var e_tb = modelBuilder.Entity<Book>();
            e_tb.Property(e => e.BookId);
            e_tb.OwnsOne(e => e.EnglishInfo);
            e_tb.OwnsOne(e => e.JapaneseInfo);
            e_tb.OwnsOne(e => e.ChineseInfo);
        }
    }

    public class Info {
        public string Title { get; set; } // a general column
        public string Author { get; set; } // a column specified HasMaxLength() by fluent API
        string Memo { get; set; } // with fluent API, a column can be implemented by a private property

        public static void OnModelCreating(ModelBuilder modelBuilder) {
            EntityTypeBuilder<Info> e_tb = modelBuilder.Entity<Info>();
            e_tb.Property(e => e.Title);
            e_tb.Property(e => e.Author)
                .HasMaxLength(100);
            e_tb.Property(e => e.Memo);
        }
    }

    public class MyContext : DbContext {
        public DbSet<Book> Books { get; set; }

        public MyContext() {
        }

        public MyContext(DbContextOptions<MyContext> options)
            : base(options) {
        }

        protected override void OnConfiguring(DbContextOptionsBuilder optionsBuilder) {
            if (!optionsBuilder.IsConfigured) {
                optionsBuilder.UseSqlite(new SqliteConnection("DataSource=:memory:"));
            }
        }

        protected override void OnModelCreating(ModelBuilder modelBuilder) {
            base.OnModelCreating(modelBuilder);
            Info.OnModelCreating(modelBuilder);
            Book.OnModelCreating(modelBuilder);
        }
    }

    public class MyContextForMigrationFactory : IDesignTimeDbContextFactory<MyContext> {
        public MyContext CreateDbContext(string[] args) {
            return new MyContext();
        }
    }

Then, I created a migration by dotnet command:

dotnet ef migrations add InitialCreate

The generated 20181206004616_InitialCreate.cs is:

    public partial class InitialCreate : Migration
    {
        protected override void Up(MigrationBuilder migrationBuilder)
        {
            migrationBuilder.CreateTable(
                name: "Books",
                columns: table => new
                {
                    BookId = table.Column<int>(nullable: false)
                        .Annotation("Sqlite:Autoincrement", true),
                    EnglishInfo_Title = table.Column<string>(nullable: true),
                    EnglishInfo_Author = table.Column<string>(maxLength: 100, nullable: true),
                    EnglishInfo_Memo = table.Column<string>(nullable: true),
                    JapaneseInfo_Title = table.Column<string>(nullable: true),
                    JapaneseInfo_Author = table.Column<string>(nullable: true),
                    ChineseInfo_Title = table.Column<string>(nullable: true),
                    ChineseInfo_Author = table.Column<string>(nullable: true)
                },
                constraints: table =>
                {
                    table.PrimaryKey("PK_Books", x => x.BookId);
                });
        }

        protected override void Down(MigrationBuilder migrationBuilder)
        {
            migrationBuilder.DropTable(
                name: "Books");
        }
    }

The problem is:

JapaneseInfo_Author has no maxLength.
JapaneseInfo_Memo is missing.
ChineseInfo_Author has no maxLength.
ChineseInfo_Memo is missing.

In short, Only the 1st Owned Entity property (EnglishInfo) can be configured by Fluent API. Configurations for 2nd (JapaneseInfo) and 3rd (ChineseInfo) are ignored.

Further technical details

EF Core version: 2.1.4, 2.2.0
Database Provider: Microsoft.EntityFrameworkCore.Sqlite 2.2.0, Pomelo.EntityFrameworkCore.MySql 2.1.4
Operationg system: Windows 7
IDE: Visual Studio 2017 15.8.7