I updated on 10/24 to 10.3 - checked today (I've always been one to wait and let others find the problems but no not this time) and all my data in the activity app is gone as many of you have experienced. I also can not update activity from my watch to my phone. I did all the normal stuff from hard resets to re-pairing my watch. My watch still won't update to the my iPhone 7 after all these steps.
I think it's time that Apple - here me Apple - allows for an easy way to downgrade to an iOS version that works. I have a backup so I can do the recovery mode restore backup crap but.....
Apple get your act together and get this fixed. While you're at it, tell your Tech Support this problem is happening. I called support today and we went through everything I had already done. Still they did not seem to know that this was happening to many, many folks. I only found out through an online article - http://www.forbes.com/sites/gordonkelly/2016/10/26/apple-ios-10-1-apple-health-l ost-data-problem/#76a76e4d2dc7
Don't keep us in the dark Apple. This should be widely known and you should have a solution today even if that means giving us an easy optional rollback to the previous version.