Type which has a owned entity and is defined as an owned entity breaks when project is upgraded to EF Core 3.1

Question

I have a Portfolio and SampleItem classes like this:

public class Portfolio 
{
        [Required]
        public Guid Id { get; set; }

        public virtual List<SampleItem > SampleItems { get; set; }

        [NotMapped]
        public List<SampleItem> Sample{
            get{
                return SampleItems;
            }
        }
}

public class SampleItems 
    {
        [Key]
        public Guid Id { get; set; }

        public string Code { get; set; }
        public string Type { get; set; }
}

When i upgraded project to .NET Core 3.1, an exception"

'The relationship from 'SampleItem' to 'Portfolio.SampleItems' is not supported because the owned entity type 'Portfolio' cannot be on the principal side of a non-ownership relationship.'

Portfolio is also defined as owned entity type in another class like this:

public class Bill
    {
        [Required(ErrorMessage = "GenId is required")]
        public string GenId { get; set; }

        public Guid PortfolioId { get; set; }
        public virtual Portfolio Portfolio { get; set; }
    }

And in my repository context class i have like this:

 public virtual DbSet<Bill> Bills{ get; set; }
 public virtual DbSet<Portfolio> Portfolios { get; set; }

I tried so many ways to solve the issue by defining ownsmany in repository context class like this:

modelBuilder.Entity<Portfolio>().OwnsMany(x => x.SampleItems);

but then i get a different exception:

'The type 'Portfolio' cannot be configured as non-owned because an owned entity type with the same name already exists.'

I cannot figure out how to solve this issue with EF Core 3.1 and we cannot rollback to .NET Core 2.x

Please help me on what i m doing wrong.

Portfolio is never configured as owned type. It is referenced in Bill class through foreign key relationship.

Here is the entire repositorycontext class

public class RepositoryContext : DbContext
    {
        public RepositoryContext()
        {
        }
        public RepositoryContext(DbContextOptions<RepositoryContext> options, IHttpContextAccessor httpContextAccessor) : base(options)
        {
            _httpContextAccessor = httpContextAccessor;
        }

        private readonly IHttpContextAccessor _httpContextAccessor;
        public virtual DbSet<Portfolio> Portfolios { get; set; }
        public virtual DbSet<Classification> Classifications { get; set; }
        public virtual DbSet<Bill> Bills { get; set; }
        public virtual DbSet<IntegrationEventLogEntry> IntegrationEventLogs { get; set; }
        public virtual DbSet<LivingExpenseSetting> LivingExpenseSettings { get; set; }

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

            modelBuilder.ApplyConfiguration(new IntegrationEventLogEntityTypeConfiguration()); //IMPOPRTANT FOR LOGGING
            var status = BillStatus.Pending.ToString();
            modelBuilder.Entity<Bill>().Property(lst => lst.Created).HasDefaultValueSql("getdate()");
            modelBuilder.Entity<Bill>()
                        .Property(p => p.Status)
                        .HasDefaultValue("Pending");
                         
             modelBuilder.Entity<LivingExpenseSetting>();
           
        }
        public override int SaveChanges(bool acceptAllChangesOnSuccess)
        {
            OnBeforeSaving();
            return base.SaveChanges(acceptAllChangesOnSuccess);
        }
    }