I have app version 1.0.95 beta and android os 8.0.0. My new phone is Samsung galaxy note 8, and the stack exchange app was installed automatically when I set the phone up (it's a warranty replacement). Notifications are turned on in the app. For one comment on one of my posts there was an indicator on the icon on my home screen for the app that I received it, but no push notification. For the comment on my answer that was posted yesterday there was no notification whatsoever except by email that I received today.
I had the same thing happen to me recently. I got a new Android phone and the app was automatically installed. (It seems that Google had backed up what I'd installed on my old phone.) I discovered that I was no longer getting push notifications.
I fixed it by going into the app itself, logging out of SE, and then logging back in again.
After logging back in again, push notifications resumed.