App turning my WiFi on? Due to the time it happens, I'm highly suspicious about WhatsApp and the Drive upload function.

I put my phone in Airplane mode before going to sleep, and this is what I see the day after.

Note that my WiFi appears as OFF when I wake up, yet the phone indicates it has been turned on for the whole night...


Battery graph and proof it turns on "ghostly" (click to enlarge)

The Drive backup starts uploading in the morning when I turn WiFi on. But I think it's around 2 AM where you see my WiFi turning on (after WhatsApp backup?)
On your screenshot, you have Wi-Fi enabled, even though you have airplane mode on

If that isn't the cause of your problem, background Wi-Fi scanning might be the cause
You can turn it off by going to Wi-Fi -> Advanced -> Scanning always available 
Or for >=6.0 Android versions, it's Location -> Scanning
That feature enables Wi-Fi even if you disable it