I recently got an Inspire 3 and noted that as it does not have its own GPS, it needs to connect to your smartphone (Android in this case) to track. I use it for cycling. When the phone screen goes black to save battery, the Fitbit app (and all others) are put to sleep. It would stop tracking in this state. It doesn't catch up either when the device is unlocked; tracking is lost and only time spent on the exercise, being a simply difference of start and end, is accurate. Adding the app to Never Sleep Apps (which requires switching it from Unlimited to Optimized) doesn't work either... all apps. are still put to sleep when the phone display is blacked out and locked.
(Thus and obviously this is completely an Android and not a Fitbit issue!)
The solution for me was to install a keep-alive app such as Screen Alive which prevents the screen blacking, disable Adaptive Brightness so Android doesn't put it back up, then for every exercise session I give this app focus and drag the screen brightness down to the bare minimum to be visible so that the phone battery lasts as long as possible, and keep the screen on putting it in a pocket etc. with the screen facing outward so it isn't activated and stays on.
By doing this I can track rides of two or more hours accurately and it only uses 2-3% of battery in this time... this is by far the best performance of any of the four Fitbits I have owned! Hope this helps.
