Some other users have reported experiencing similar issues when travelling for periods of time at slower speeds via methods other than walking or running (for example: riding in golf carts, on mopeds or on horses).
It may help to calibrate your Apple Watch, including resetting any existing calibration data (this process can be equally as beneficial for Series 3 models):
Calibrating your Apple Watch for improved Workout and Activity accuracy - Apple Support
Activity-related estimations also depend, in part, on your personal information. To check that this is accurate - and to update it over time:
On your iPhone, in the Watch app, go to: My Watch (tab) > Health > Edit - tap and adjust items, then tap on Done.
It may also help to check your Location Services settings:
On your iPhone, go to: Settings > Privacy > Location Services:
- Check that Location Services (the main setting at the top) is turned on.
- Enable options under System Services including Motion Calibration & Distance, Wi-Fi Networking and Mobile Network Search.
Otherwise, you may wish to submit feedback to Apple:
Feedback - Apple Watch - Apple