I would tend to agree with Kyle W. I've had the Nexus 6 for a few months now and I never had trouble connecting and reconnecting to WiFi networks, but as of the latest OTA update, I've started experiencing the exact same problems, viz. my phone doesn't recognize saved WiFi networks or any nearby WiFi networks for that matter.
I think I've been noticing the same sort of activity this past week, but it's inconsistent. I have noticed that my phone is on T-Mobile cellular when it should be connected to T-Mobile WiFi Calling because it's all of 18" away from my router and all the other WiFi rigs in the place are connected to the same router. Sometimes a restart works; sometimes opening up Network SIgnal Info also jogs the phone bak to WiFi Calling. But not every time. I'll try toggling Airplane Mode if I see the same thing happening tonight.
I can remember seeing similar activity just before I became aware that Google Connectivity Services was a thing, which for me was shortly after the T-Mo Wifi Calling update, so I'm also willing to believe it's the culprit again. Although there's also an odd difference this time. Before, if I checked Network Signal Info, I would see that my Mobile signal was a solid 100% while my WLAN signal was typically more like 80-85% whether the phone claimed to be on "T-Mobile" or "T-Mobile WiFi Calling." Now when I check the same app, my Mobile signal is always "disconnected" if I have a WiFi connection, but more often than not the display indicates that the phone
is nevertheless connected to "T-Mobile" cellular and
not to "T-Mobile WiFi Calling."