I'm having this same problem. Setting the app to 'Don't optimize' did not resolve it. Also enabled 'unrestricted data access' and all notification settings are on (Allow interruptions, on lock screen, override DO Not Disturb).
I've had multiple times(in the past couple weeks) where things have went down on my server and I haven't received a single notification.
It started when I got a google pixel and installed pulseway.
I've tried uninstalling and reinstalling, doesn't resolve anything. Also wasn't prompted for any permissions.
I've been using Pulseway for like 3 years now, but it's feeling really buggy right now.