Hello @TommiTitbit and welcome to the Community. The temperature setting for the weather is in the Weather app. Please tap on your device/battery icon in the upper left of your Fitbit app > Sense 2 > Gallery > scroll all the way down to the bottom of the page and tap on Weather. Now tap on Settings and change the units to Celsius.
The temperature setting in the Date, time & units settings is for the skin temperature variation, a common source of frustration as it lacks explanation.
Laurie | Maryland
Sense 2, Luxe, Aria 2 | iOS | Mac OS
Take a look at the Fitbit help site for further assistance and information.