The next update to iOS 12 will fix many annoying issues users are experiencing.
Apple has addressed some issues people are having in iOS 12, to do with the Activity app.
If you've updated to iOS 12 and you are missing some awards and challenges from the Activity app, it is not you, and is a current issue in iOS 12. Specific challenges and awards that may not appear include:
Thanksgiving Day Challenge 2016
Mother’s Day Challenge 2017
Veterans Day Challenge 2017
Thanksgiving Day Challenge 2017
China National Fitness Day 2018
Elsewhere, some users have reported that FaceTime Live Photos is gone (not intentionally) and also traffic data for Apple Maps isn't showing up either. There are also claims that battery life isn't good for some devices, so this may be fixed.
iOS 12 has been one of the smoothest versions of iOS ever released, with barely any bugs and issues.
Apple have said they are aware of these issues, and will fix them in a software update to iOS 12, most likely iOS 12.0.1 which could come out by next week.