Delay in notifications

Since the last update of 15.6.1, my iPhone had experienced a delay in receiving notification. I only see the received message in Whatsapp, messenger etc only if I open the apps. It never happened before. I had done several troubleshooting such as turn off focus mode(DND), disable low power mode, disable low data mode for all connection, turn on notifications, switched off scheduled summary, allow notification for whatsapp and messeger etc. But nothing works. Is it some kind of bugs, or it is something I can resolve via setting by some other ways? Hope someone can advice.