I just got an iPhone X a couple of days ago and was unable to sync via bluetooth. I contacted the online chat with Fitbit and was told it was not compatible yet. Just now I decided to uninstall the Fitbit app from my phone and re download. After I logged in to the app, it said it would like to pair with my Fitbit and it worked. All my stats from the tracker synced up as well.
Hope this helps.
