UserManager.CreateAsync无法将用户保存至数据库的问题求助

我尝试了多种方法排查这个问题：调用UserManager.CreateAsync(user, request.Password)后执行_context.SaveChangesAsync()，数据库表没有新增任何记录，而且SaveChangesAsync()返回0（表示无数据受影响）；但如果先调用CreateAsync创建用户，再执行_context.Users.AddAsync(user)后调用SaveChangesAsync()，用户数据就能正常保存到数据库。补充说明：user.Id已经有值，CreateAsync和SaveChangesAsync都没有抛出异常（已添加try-catch捕获）。

相关代码 #

ApplicationDbContext.cs #

public class ApplicationDbContext : IdentityDbContext<User>, IApplicationDbContext
{
    public ApplicationDbContext(DbContextOptions options) : base(options)
    {
    }

    public DbSet<ExternalApplications> Applications { get; set; }

    public override async Task<int> SaveChangesAsync(CancellationToken cancellationToken = default)
    {
        return await base.SaveChangesAsync(cancellationToken);
    }
}

PersistenceDI.cs #

public static class PersistenceDI
{
    public static IServiceCollection AddPersistenceLayer(this IServiceCollection services, string connectionString)
    {
        services.AddDbContext<ApplicationDbContext>(options => options.UseSqlServer(connectionString));

        services.AddIdentityCore<User>().AddEntityFrameworkStores<ApplicationDbContext>().AddUserManager<UserManager<User>>();

        services.AddScoped<IUserRepository, UserRepository>();

        return services;
    }
}

User.cs #

public class User : IdentityUser
{
    public string FirstName { get; set; }
    public string LastName { get; set; }
}

CreateUserCommandHandler.cs #

public record CreateuserCommand : IRequest<string>
{
    public string Email { get; set; }
    public string Password { get; set; }
    public string FirstName { get; set; }
    public string LastName { get; set; }
}

public class CreateUserCommandHandler : IRequestHandler<CreateuserCommand, string>
{
    public IApplicationDbContext _context;
    public UserManager<User> _userManager;

    public CreateUserCommandHandler(IApplicationDbContext context, UserManager<User> userManager)
    {
        _context = context;
        _userManager = userManager;
    }

    public async Task<string> Handle(CreateuserCommand request, CancellationToken cancellationToken)
    {
        User user = new User
        {
            Email = request.Email,
            FirstName = request.FirstName,
            LastName = request.LastName,
            UserName = request.FirstName + " " + request.LastName + "eqweetwtweywe",
        };

        await _userManager.CreateAsync(user, request.Password);
        await _context.SaveChangesAsync(cancellationToken);

        return user.Id;
    }
}

内容的提问来源于stack exchange，提问作者Vladyslav