Solution to https://discussions.unity.com/t/ontriggerenter-not-firing-if-i-instantiate-a-gameobject-completely-inside-anothers-trigger/35942

ColliderTwiddler.cs

using System.Collections.Generic;
using UnityEngine;

public static class ColliderTwiddler {
  static List<Collider> colliders = new();

  /**
   * This is a hack to force the colliders to trigger OnTriggerEnter/etc.
   * Call this after instantiating an object or adding a rigidbody to it.
   */
  public static void Twiddle(GameObject gameObject) {
    colliders.Clear();
    gameObject.GetComponentsInChildren(colliders);
    foreach (Collider collider in colliders) {
      bool val = collider.enabled;
      if (val) {
        collider.enabled = false;
        collider.enabled = true;
      }
    }
  }
}