With the increasing number of Fitbit devices that track heart rate data, Fitbit changed how they track activity time for some devices.
For these newer devices, Fitbit introduced Active Zone Minutes. These minutes are based on a user's heart rate data and the amount of time they are in the fat burn, cardio, or peak heart rate zones.
The devices that track Active Zone Minutes are:
Fitbit Charge 4
Fitbit Inspire 2
Fitbit Ionic
Fitbit Luxe
Fitbit Sense
Fitbit Versa
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.
Currently, ONLY Active Minutes are supported by the Fitbit API. Active Zone Minutes are not supported by the Fitbit API.
As a result, for users wearing the devices listed above may show 0 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