Try work around above. I get instant notification on gmail to reply your text. Be detail. What workaround have you done? If facebook instagram notifications showen again and again as you mention, it might be the app problem. Actually here, we are trying to fix PUSH notification. Not the notification. As the app showing notification again and again, its not related to push notification. It might be the app itself. Try updating the apps or updating to the latest firmware.
I disabled all Device Health things using ADM pm disable/pm uninstall command and restarted ( https://www.xda-developers.com/disable-system-app-bloatware-android ). Now notifications still got blocked when phone was locked.
Then I installed Wakelock power manager on playstore and set it to partial wakelock. Now it's working perfectly!! (Finally!)
Many thanks to all of you!
Doesnt matter you can still use it even if its older version. The one you downloaded also can be use dont worry. The purpose of the app are still the same. Just use partial wakelock and you are good to go. Dont optimise this app and set it to autostart.