I found the problem if you no longer receive messages in standby mode, but only when you open the app.
After the update, by my phone Android 7.0, there is a new item in the menu: background task clear.
In this menu you can therefore exclude apps from the deep sleep mode. With this new option, all your apps go in default deepsleep modus and you get
no whatsapp messages (and other message notifications like your alarm or email....) more if your phone has gone into standby mode. Ticking the square box by whatsapp (or any other app) and you will receive the messages in standby mode.
Tell it further!

No whatsapp and Facebook notifications when phone is blocked since did actualization to android 7

Not any notification from Facebook messenger, WhatsApp...
Not even widget activities.
Lot of bugs

Try to turn off background task killer.

Try to close bakkground task killer in settings.

Try to close background tasks killer In settings.

I found the solution!
It must be enabled "Background task clear."
If you want to have all the applications active in the background you need to enable this option and do not select any application (in the list of applications).
This option is disabled as kills everything in the background.