I am using the WhatsApp for Desktop app on my Windows 10 PC. It prompts me to rescan the QR code every time my phone's display turns off - this is in spite of the fact that I checked the 'keep me signed in' box on the desktop app before scanning the QR code.
Data saver and battery saver are turned off. I have also explicitly allowed in battery saver for whatsapp to run in the background. I am connected to a wifi network (with my pc being on the same network) and have a celluar connection.
My phone is the 640 LTE with Windows Phone 8.1 Update 2.