I have also been facing this issue. It always calls the onAdFailedToLoad callback but when I add a delay it shows fine. I am not sure if adding a delay in showing App Open Ad allowed or would it cause "Modified Ad Behavior" policy violation.
By delay I mean:

`override fun onStart(owner: LifecycleOwner) {
super.onStart(owner)
addDelay()
}

private fun addDelay() {
CoroutineScope(Dispatchers.Main).launch {
delay(100) // Delay for 2 seconds (2000 milliseconds)
// Code after delay
println("Delay finished.")
currentActivity?.let { appOpenAdManager.showAdIfAvailable(it) }
}`