In my MyFirebaseMessagingService, I create a notification (even when the app is currently in foreground):

intent = Intent(applicationContext, MyActivity::class.java).apply {
    flags = Intent.FLAG_ACTIVITY_NEW_TASK
}

val pendingIntent = PendingIntent.getActivity(applicationContext, 0, intent,
    PendingIntent.FLAG_ONE_SHOT or PendingIntent.FLAG_IMMUTABLE)

notificationBuilder = notificationBuilder
    .setContentIntent(pendingIntent)
...

Now I believe I have tried everything in my power for this new activity to trigger an animation:

(1) set animation via bundle with pending intent:

val animationBundle = ActivityOptions.makeCustomAnimation(
    this,
    R.anim.slide_in_right, // Animation for new activity entering
    R.anim.slide_out_left // Animation for current activity exiting
).toBundle()

val pendingIntent = PendingIntent.getActivity(this, 0, availabilityDealIntent,
                    PendingIntent.FLAG_ONE_SHOT or PendingIntent.FLAG_IMMUTABLE, animationBundle)

(2) via onCreate of the new activity (works perfectly when I start the activity via startActivity, but not when I click the notification):

if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.UPSIDE_DOWN_CAKE) {
    overrideActivityTransition(OVERRIDE_TRANSITION_OPEN, R.anim.slide_in_right, R.anim.slide_out_left, 0)
} else {
    overridePendingTransition(R.anim.slide_in_right, R.anim.slide_out_left)
}

(3) via app theme animation (android:windowAnimationStyle):

<item name="android:windowAnimationStyle">@style/LeftRightAnimation</item>

<resources>
    <style name="LeftRightAnimation" parent="@android:style/Animation.Activity">
        <item name="android:activityOpenEnterAnimation">
            u/anim/slide_in_right
        </item>
        <item name="android:activityOpenExitAnimation">
            u/anim/slide_out_left
        </item>
        <item name="android:activityCloseEnterAnimation">
            @anim/slide_in_left
        </item>
        <item name="android:activityCloseExitAnimation">
            @anim/slide_out_right
        </item>
    </style>
</resources>

(4) via app theme transition:

<item name="android:windowActivityTransitions">true</item>
<!-- specify enter and exit transitions -->
<item name="android:windowEnterTransition">@anim/slide_in_right</item>
<item name="android:windowExitTransition">@anim/slide_out_left</item>

Some additional information:

• no launch mode set in Manifest
• doesn’t matter which activity is currently active while I click the notification, animation never happens
• doesn’t matter whether the app is in foreground or background while clicking the notification
• onCreate() of the new activity gets only called once
• animations work perfectly fine in every other case

I’m at the point of giving up.