Why my entity primary key shows to be modified after updating entity?

I have a class ApplicationUser that inherits from IdentityUser. I added the property LastLoggedIn which is OffsetDateTime datatype and I modify it with every request to the API. Also, I am using Audit from entityframework plus to log user add/update/delete operations and this is how I came across this issue. every time I update the ApplicationUser entity's LastLoggedIn property I can see that the Id property has been modified as well, showing same old and new value. Why my primary key is being modified here?

I posting the relevant code,

public class ApplicationUser : IdentityUser {
        [Required, StringLength(50)]
        [Display (Name = "First Name")]
        public string FirstName { get; set; }
        public OffsetDateTime? LastLoggedIn { get; set; }
}

Method calling the entity, since the class inherits from IdentityUser, I am using UserManager

 public async Task UpdateUserLastActivityDateAsync (string userId, string timeZone) {
            var user = await userManager.FindByIdAsync(userId);
            user.LastLoggedIn = timeZone.GetInstantOffsetDateTime();
            await unitOfWork.CompleteAsync(); //  = context.SaveChangesAsync();
        }

I tried the following but I get an exception,

builder.Entity<ApplicationUser>()
        .Property(e => e.Id)
        .ValueGeneratedOnAddOrUpdate()
        .Metadata.BeforeSaveBehavior = PropertySaveBehavior.Ignore;

The property 'Id' cannot be configured as 'ValueGeneratedOnUpdate' or 'ValueGeneratedOnAddOrUpdate' because the key value cannot be changed after the entity has been added to the store.