Verify why object is only garbage collected on second garbage collection run

For learning purposes I am currently experimenting with Garbage Collection (Pixel 2, Android 10 -> ART). My observation for the following experiment is that CountActivity is never deleted on the first explicit garbage collection run, but only at the second explicit garbage collection run. I want to understand why it isn't deleted on the first run. I assume it's due to some kind of generational garbage collection, but I want to verify it. How can I "watch" what the garbage collection does? E.g. is it possible to see the internal generations in which the garbage collection groups the heap?

My test example is as follows:

1. MainActivity, which shows a Button "Show CountActivity"
2. CountActivity which is shown when the Button "Show CountActivity" in the MainActivity is pressed.
3. I end the CountActivity using my system back key

After this, I force an explicit garbage collection run, capture a heap. CountActivityis still there. After forcing a second explicit garbage collection run and again capturing a heap, the CountActivity is gone. Now I still want to understand why it takes two garbage collection runs.

MainActivity

class MainActivity : AppCompatActivity() {

  override fun onCreate(savedInstanceState: Bundle?) {
    setTheme(R.style.AppTheme)
    super.onCreate(savedInstanceState)

    setContentView(R.layout.activity_main)

    buttonStart.setOnClickListener {
      showCountActivity()
    }

  }

  private fun showCountActivity() {
    val intent = Intent(this, CountActivity::class.java)
    startActivity(intent)
  }

}

CountActivity

class CountActivity : AppCompatActivity() {

  override fun onCreate(savedInstanceState: Bundle?) {
    super.onCreate(savedInstanceState)
    setContentView(R.layout.activity_count)
  }
}