The issue is not related with Deep Sleep function.
The issue can occur at any time of the day while using "Whatsapp Web" on your computer and your mobile is connected to the WiFi.
When the issue occurs, the Wi-Fi is completely disconnected, the mobile data is completely disconnected and the bluetooth is completely disconnected.
This issue only happens using Whatsapp Web, if you don't use it this issue never happens even if the phone is many hours with the screen off.
Regards,