WiFi disconnects in doze mode Nexus 5x and Pixel with project fi. No antivirus installed.
OS 8.1.0

Regularly puts me on LTE connection both at home and in the office.

How do I force WiFi to always connect to known networks rather than surprising me every time that I just watched a video and used tons of data?

Is this related to the removal of Wi-Fi sleep options? And what "Sleep options are not relevant any more." means that the relevant commit message (via reddit) says?
Customer service explained me that my Project Fi SIM card is expected to kick me off from free Wi-Fi whenever it decides that the LTE connection is stronger. He said the customer should every single time manually put himself to airplane mode as a remedy. Complete bullshit esp. given that fact that it loses Wi-Fi in sleep even if data is off so waking the phone would not give me any connection, until I would force a reconnect by opening settings.

Finally, a device update from March 5 fixed it, so now my Wi-Fi stays on.