I have three Android devices.
Lenovo A7000 with Stock ROM Lollipop 5.0 HTC Desire 620G with Stock ROM KitKat Samsung galaxy Tab 2 with custom ROM (Resurrection ROM) Lollipop 5.0.2.
In all these devices the WhatsApp sleeps frequently, when the screen is Off. I mean when I on the device screen and use WhatsApp I am getting messages normally. But when the screen is off after a short span of time, it stops receiving messages. I need to turn on the screen and open WhatsApp to start getting the messages. Then bunch of messages will be received.
I am facing this problem in all the three devices in both Wi-Fi and Mobile networks. My Wi-Fi network is fast enough (40 Mbps) to receive the messages at instant and mobile networks are decently fast (>3 Mbps).
In WhatsApp Web I am getting Phone Not Connected yellow messages when I turn the screen off. After researching in the Internet I learnt this may happen when the device enters power saving mode or when Background data is restricted. But in my devices I never turn on power saving mode and never restrict background data too.
Any other reasons? For this reason I have changed screen timeout to 30 minutes and left the WhatsApp open. Is this an app bug or some other problem?