S8 - wifi disconnects on sleep

[elParaguayo]

I thought I'd post here in case anyone has managed to fix this issue (although, based on my wider searches, I'm not sure anyone has yet).

The issue I'm having is that the wifi disconnects when my phone sleeps (i.e. the screen is off for a number of minutes). This is problematic where I'm downloading data in the background.

I've checked in the Wifi settigns and made sure the "Keep Wi-Fi on during sleep" is set to "Always".

I've turned off smart network switch too.

Is this a bug in the phone or a wider Android issue?

It's quite frustrating for me as there are a number of reasons while having the wi-fi constantly connected is very important for me.

[elParaguayo]

It actually looks like the phone has 2 whitelists:

Settings-->Device Maintenance-->Battery-->Unmonitored Apps (this is the one I used originally with no success)

The other is:

Settings-->Apps-->[Three dots]-->Special Access-->Optimise Battery Usage. You can then select "All apps" and uncheck particular apps. I suspect this is the list that the adb command from my earlier post fixed.

Hopefully this will help other people.