I've previously had 4 different Blackview models, but this annoying thing happens to me also, only on A10.
I've tried everything from above, but no success.
Notifications from the native apps stay (Gmail, Maps, Play Store...), but all other apps (Facebook, Messenger, Viber, Whatsapp, Instagram...) get killed in the background (even if minimized by home button) and notifications destroyed after the phone locks and the screen is off for only 5-6 seconds.
For example: If Facebook Messenger is started and running, it stays on. But if just minimized, or a message is received, or a chat head is left running, 5-6 seconds after the phone is locked, the app is killed and the notification is removed.
After that, no messages are received until the Messenger is started again and it takes about 10 seconds for the app to start again, while, if the phone isn't locked yet, the app starts almost instantly.
It's like Developers: 'Don't keep activities' and 'No background processes' are active, but they are not...