You are right. That sometimes works. But what I've noticed, maybe it's some issue with my set up - not sure, that often the WhatsApp desktop/web app doesn't connect for 1-2 minutes even after I have unlocked (or woken) my phone - so I just click on WhatsApp app on phone anyway and let it reconnect. Android and iOS both are pretty strict on background tasks these days. I hope battery tech catches up with the speed at which apps' resource appetite is growing.