Right now I test 2 devices (Android 10 and 8) parallel (via subscription to the same topic):
Android 8: All good
Android 10: All good, too (but more options)
It seems that the messages ARE delivered on time, but after this it is important to set the notification importance to "HIGH":
n.Initialize("default", Application.LabelName, [B]"HIGH"[/B]).AutoCancel(True).SmallIcon(icon)
otherwise the notification is created but it is handled less agressive and the phone decides to inform "later".
On Android 10 go to the Notification options -> your app -> there are different profiles created automatically for each importance. Good to know.