Artifacts used:
androidx.appcompat:appcompat:1.1.0
androidx.fragment:fragment:1.3.0-alpha03

Theme used: Theme.AppCompat.Light.DarkActionBar
Devices/Android versions reproduced on: Pixel 4 (RP1A.200115.001.C1)

Recreating this issue per comment on b/37096307. This may just be working as intended at this point, but it is still reproducible with the latest libraries. The behavior change surfaced between 23.1.1 and 23.2.0 (legacy support library artifacts).

Minimal code to reproduce is below. Clicking on the displayed button will cause a crash, as the prior expectation was that in onAttachFragment(), the FragmentManager will contain the Fragment that is passed as a parameter to onAttachFragment(), but this is no longer the case.

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import android.graphics.Color
import android.os.Bundle
import android.view.LayoutInflater
import android.view.View
import android.view.ViewGroup
import android.view.ViewGroup.LayoutParams
import android.widget.Button
import android.widget.ImageView
import androidx.appcompat.app.AppCompatActivity
import androidx.fragment.app.Fragment

private const val FRAGMENT_TAG = "tag"

class MainActivity : AppCompatActivity() {
override fun onCreate(savedInstanceState: Bundle?) {
super.onCreate(savedInstanceState)

if (savedInstanceState == null) {
supportFragmentManager
.beginTransaction()
.add(android.R.id.content, StartingFragment(), FRAGMENT_TAG)
.commit()
}
}

override fun onAttachFragment(fragment: Fragment) {
super.onAttachFragment(fragment)

val fragmentByTag = supportFragmentManager.findFragmentByTag(FRAGMENT_TAG)
if (fragment != fragmentByTag) {
throw IllegalStateException("Expected $fragment but found $fragmentByTag")
}
}

fun replaceFragment() {
supportFragmentManager
.beginTransaction()
.replace(android.R.id.content, ReplacementFragment(), FRAGMENT_TAG)
.addToBackStack(null)
.commit()
}

class StartingFragment : Fragment() {
override fun onCreateView(
inflater: LayoutInflater,
container: ViewGroup?,
savedInstanceState: Bundle?
): View? {
return Button(requireContext()).apply {
layoutParams = LayoutParams(LayoutParams.MATCH_PARENT, LayoutParams.WRAP_CONTENT)
text = "Replace fragment"
setOnClickListener {
(requireActivity() as MainActivity).replaceFragment()
}
}
}
}

class ReplacementFragment : Fragment() {
override fun onCreateView(
inflater: LayoutInflater,
container: ViewGroup?,
savedInstanceState: Bundle?
): View? {
return ImageView(requireContext()).apply {
layoutParams = LayoutParams(LayoutParams.MATCH_PARENT, LayoutParams.MATCH_PARENT)
setBackgroundColor(Color.BLUE)
}
}
}
}