This morning, I was using my Pixel phone to search some items and social media. I also saw an email from Verizon that I had 34% of my data remaining with 6 days to go, so I thought, That's good. Six hours later, I get another email from Verizon saying that I had 12% of my data remaining. I was shocked. I checked the phone to realize that I had no WiFi connection to my home router. I tried to connect, but no luck. I restarted the phone and the connection resumed. Do I have to check every time I use my phone to see if the WiFi is connected? I checked and my Cellular Connection was "on", so I immediately turned that "off". What a pain.
I have a Pixel as well, and have noticed that occasionally it drops the wifi. It seems to happen after some app or another has updated - resetting the wifi, or using data rather than wifi to backup or update things. Then I need to turn the wifi back on and reconnect.
I've learned to NOT auto update apps, but get notifications if an update is available. Then go in and manually run the updates, check settings for any that do any backing up (Google photos got me awhile back, backing up all my photos over cellular data!) or may have changed your default settings. It doe stake more time, and is tedious, but may prevent data surprises.