I have Nokia 5.1 Plus with Android Pie.
Now, since a few days, I'm facing this irritating problem. When the WhatsApp is open, it works fine. You receive push notifications, and messages are not delayed.
When you minimize it (i.e., open other apps) and leave the phone away, I don't notice any problem for 10-15 minutes. Even when screen is locked, I receive WhatsApp notifications.
After 10-15 minutes, WhatsApp kind of freezes. When I try to send message from other phones to my phone, I see nothing. When I use phone and open WhatsApp (which wasn't closed, just minimized), I see it takes 2-3 seconds to fetch pending messages. So it kind of stops.
I have removed all battery optimizations settings for this app. Did reset of the app, still can't solve this issue. How can I fix it?