Some users have noticed that after saving an activity that was tracked using the phone and watch connected, that info from their activity changes once it's saved. This is a major bummer for those users, but the good news? Our team is grinding away to get this bug fixed as I type this! If you run into this problem let us know, so we can update you when it's fixed.
In the meantime, one possible solution to avoid this in the future is to make sure you control (start, stop, save etc.) any Watch + Phone trip from the watch. That usually assures the trip will be tracked accurately.
Important Note: Always make sure "Watch + Phone" is selected before starting a connected activity with the devices.