have you tried:Hello im facing the same problem as yours. No notification from mate 20 pro while screen off after 30 seconds. But the worst thing is, i need to reset wifi connection to get my notification come in. They do not come if i turn my screen on.
Im currently using Push Notification Fixer from Play store apps
This application may help you to get notification while screen off. But only periodically 5-10 minutes. Not instant notification.
for example for whatsapp:
-settings/notifications/whatsapp/ check if "allow notifications" is ON
-settings/apps/apps/three dots up right/special access/battery optimization/ whatsapp/ "dont allow battery optimalization"
-settings/battery/app launch/whatsapp/"manage manually"