Hi
There has never been a known issue with Apple Watch tracking treadmill or elliptical workouts and it is not necessary to carry your iPhone or have it with you.
Be sure to record your workouts via the Workout app, choosing the activity type that most closely matches your workout. This enables Apple Watch to use the most appropriate sensors and data sources when estimating the results, including using tailored calorie calculations for preset activity types.
During general daily wear and when using the Workout app to record workouts when GPS data is either not relevant or not available - including Indoor Run and Indoor Walk workouts on a treadmill and Elliptical workouts - the Activity app and Workout app track your arm motion (measured by the accelerometer) when estimating results including steps, calories and progress towards the Exercise goal.
For the best results, allow the arm on which you are wearing your watch to swing naturally when walking or running and to move naturally as expected for other workout types (where relevant to the activity type).
Calibrating your watch can improve the accuracy of estimations made by Activity app and Workout app. Calibration teaches your watch how your arm movements relate to your stride lengths at different speeds when you are walking and/or running. It does this - during outdoor walks and/or outdoor runs - by comparing data from the accelerometer with GPS / Location Services data.
More information and instructions for calibrating your watch are available via the link below. The article also includes instructions for resetting existing calibration data and starting afresh. Resetting your calibration data will not erase your Activity history.
For the best performance from the heart rate sensor (data from which is used when estimating calories) during workouts, be sure to wear your watch snugly on the top of your wrist. Apple suggests that you consider tightening the band before starting a workout and loosening it again afterwards:
Your heart rate. What it means, and where on Apple Watch you’ll find it. - Apple Support
More information:
Calibrating your Apple Watch for improved Workout and Activity accuracy - Apple Support
Use the Activity app on your Apple Watch - Apple Support
Use the Workout app on your Apple Watch - Apple Support