My controller will disconnect during the middle of gameplay. It dropped on me.during a squad in PUBG, and then again when playing ACO. It happens daily, and it's not due to battery life as the controller can be plugged in to the USB-C charger while I'm playing. This gets frustrating as I've reported it to Google through the Feedback option in the App, but have gotten zero response.
If you can, check any logs on your wifi access point/router to see if there is a reason for the drops. It's "probably" because of some wifi interference. You can experiment with changing the channel(s) your wifi is operating on to see if things get better or worse.
What type of router are you using . Was having same issues upon starting using stadia . My controller would randomly disconnect leaving me stranded in games. And it would take a restart to reconnect. Upon further investigation. My CCU was hardwired my controller was wireless obviously. But i had the same problem regardless of the way i connected the CCU . So i found out that my router had the same SSID for both frequencies and dynamically changed to suit needs of controller. In doing so the controller would disconnect. I bought another router(my ISP provided router was locked due to my wireless cable setup.) With my new dual band router i setup my 5ghz band to be static so once my controller connected to the network it stayed there until it turned off. Haven't had one issue since. Hope this method works for you or at least allows you to troubleshoot your current router setup