Subentity not persisting in EF Core?

Question

ASP.NET

An entity comes to the controller

public class IdentityUser
    {
        public int Id { get; set; }
        public string UserName { get; set; }
        public string Login { get; set; }
        public string PasswordHash { get; set; }
        public int RoleId { get; set; }

        public virtual List<UserDivision> UserDivisions { get; set; }
  }

    public class UserDivision
    {
        public int IdentityUserId { get; set; }
        [JsonIgnore]
        public IdentityUser IdentityUser { get; set; }

        public int DivisionId { get; set; }
        [JsonIgnore]
        public Division Division { get; set; }
    }

controller code

[HttpPut("{id}")]
        public async Task<IActionResult> PutIdentityUser([FromRoute] int id, [FromBody] IdentityUser identityUser)
        {
            if (!ModelState.IsValid)
            {
                return BadRequest(ModelState);
            }

            if (id != identityUser.Id)
            {
                return BadRequest();
            }
            _context.Entry(identityUser).State = EntityState.Modified;
            try
            {
                await _context.SaveChangesAsync();
            }
            catch (DbUpdateConcurrencyException)
            {
                if (!IdentityUserExists(id))
                {
                    return NotFound();
                }
                else
                {
                    throw;
                }
            }

            return NoContent();
        }

the UserDivisions entity is not stored in the database.