I think I have the answer.
Besides disabling Wifi and turning off the "automatically turn-on Wifi", there's a setting in the Apps & Notifications section.
For the Pixel 2 XL phone, purchased through Google.com (should apply to the carrier one too).
Go to Settings > Apps & Notifications
At the very bottom, expand Advanced
Then at the very bottom, expand "Special App access"
There's a section called "Wifi Control"
In there, go to each app and de-activate Wifi control.
I think there is where individual apps may be turning on Wifi itself, even though you have it turned off at the system level.
good luck!