...
The devices that track Active Zone Minutes are:
Fitbit Charge 4 & 5
Fitbit Inspire 2
Fitbit Ionic
Fitbit Luxe
Fitbit Sense
Fitbit Versa series
All other Fitbit devices track the traditional Active Minutes which are based on the user's movement. Active Minutes are earned when the user has at least 10 minutes of continuous moderate-to-intense activity.
...
As a result, for users wearing the devices listed above may show 0 different values for activity minutes in the Validic API.
Fitbit's full documentation on these features can be found here:https://help.fitbit.com/articles/en_US/Help_article/1379.htm
https://help.fitbit.com/articles/en_US/Help_article/1379.htm