And English Australia / English UK won't display Celsius by default without web permissions? (Granted, that would also mess with the display of calendar dates.)
I think I understand what's happening here. If I click the temperaure on launcher, it opens the Google Search app querying for "weather" - here it is in celsius.
Temperature units preference is saved in Google Search app. Pixel launcher, even though it shows the news cards from Search app, cannot show temperature in the units of my choice unless I give it the "Web and App activity" permission. Just Google things.
This is pretty surprising - what locale are you using?