For some reason quite a few times when I open the Zwift iPhone app it has switched to Imperial units from metric. It’s not a big deal to swap it back but don’t know why it would remember these settings.
That is horrible 
Seriously though, I have never seen this happen. I just tried opening and closing the Zwift app 5x w/out issue.
What version of iOS?
Maybe delete Zwift and reinstall?
I know it’s such a terrible thing to happen!!
I’m on the latest version and it happened on the version before that. I don’t know if perhaps its because I have the companion app in Imperial so that I can enter my weight in pounds (to be more accurate).