Why is my Samsung Galaxy S5 disconnecting from Wifi after 30 minutes of sleeping/idling?

[AC Question]

I have a Samsung Galaxy S5 which I use mainly on Wifi. It connects to Wifi fine, it receives incoming messages fine and basically works fine.
The problem starts when the phone is left idle. After about 30 minutes, I don't receive any notifications from apps like Whatsapp or Facebook. When I activate the screen, however, all missed notifications pour in (often with wrong timestamps).

It seems that the phone disconnects from the Wifi data stream after ±30 minutes of idling.

IMPORTANT: 'Keep wifi on during sleep' is set to 'Always', so it shouldn't sever the data stream. Also, all accounts are set to auto sync.

It is a problem, because the whole point of push notifications is that you receive the messages when they come in, and not up to 3 hours later, as has happened to me.

I have been unable to find a solution on my own, and was hoping that someone here could help me out. I will be following this question and providing additional information where needed. Thanks in advance.

 

[B. Diddy]

Welcome to Android Central! Go to Settings>Wi-fi, tap Menu>Advanced, and make sure Wi-fi Optimization is unchecked. This setting is actually to save battery, not to improve your wi-fi reception. Perhaps it's causing wi-fi to turn off after it's been asleep for a while as a way to save battery.

 

[RAKMO LITAP]

I was facing similar problwm and after lots of digging i noticed that my wifi was being turned off by a battery saving app (avast)which i installed in past. After uninstalling it the problem was fixed.
Also my friend had similar problem and in his case it was old version Macfee antivirus app which was turning the wifi off