Entity framework core lazy loading not performed when inserting new instances

I have two classes:

• Campaign which references a class customer:

  public class Campaign
  {
      [Key]
      [Required]
      public int id { get; set; }
      public int? CustomerId { get; set; }
      [ForeignKey("CustomerId")]
      public virtual Customer customer { get; set; }
  }
  

• And Customer:

  public class Customer
  {
      [Key]
      [Required]
      public int id { get; set; }
      [Required]
      public string name { get; set; }
      [Required]
      public double turnover { get; set; }
      public virtual ICollection<Campaign> campaigns { get; set; }
  }
  

Here's is an insert method:

async Task<Campaign> ICampaignRepository.InsertCampaign(Campaign campaign)
{
    try
    {
        _context.Campaigns.Add(campaign);
        await _context.SaveChangesAsync();
        return campaign;
    }
    catch (Exception)
    {
        throw;
    }
}

I'm using Microsoft.EntityFrameworkCore.Proxies package for lazy loading.

After adding a campaign instance having a customerId, the customer is not lazy loaded in the inserted object. Please note that I tried to fetch the campaign by id before returning it, but the problem persists, and I want to avoid loading the customer explicitly.

Lazy loading is perfectly working when performing fetch operations on existing records.