I have the China set 3/32GB and unfortunately i'm facing the delayed (or none) notifications from WhatsApp/Airbnb etc unless I open up the app.
tried setting these apps to autostart and disable battery saver but not working. seems like a common issue but only affecting certain users only.
my mi max2 on Android 7.1 has no such issues (as I migrated from Mi Max 2 to RN5)
lock the important apps in your ram.