According to the complaints of some Fitbit users, it appears that after their phones have been updated to Android Pie, their Fitbit devices have stopped syncing with the app. Users discovered this after installing the update and have taken to the forums to voice their complaints.
The good news is that Fitbit is aware of the issue and a fix is already in the works. In a post on the forums, a Fitbit moderator replied by saying, “Hey Everyone! Our team is still working on implementing a longterm fix for syncing with Android devices on Pie (9.0).” They also revealed that they have released an update for the app that should also bring about some improvements related to syncing, so maybe you could check that out to see if it helps.
Not all Fitbit users are affected, thankfully, but in case you are, then check out the latest version of the app which will hopefully resolve any problems you might have. If not then you will need to wait for the longterm solution that Fitbit has promised is on its way.