Since you’ve already updated and restarted both devices, tightened the strap, and reset calibration, I understand why this is frustrating—especially when the problem comes and goes.
A few other things to try on your Apple Watch Ultra:
- Check your personal details in the Watch app -> My Watch -> Health ->Health Details. Make sure your height, weight, age are correct.
- Calibrate outdoors: In a flat, open area with good GPS reception, start an Outdoor Walk workout and walk at your normal pace for about 20 minutes.
- Compare step counts during a specific walk with your iPhone in your pocket. Check the Health app’s step data source to see which device recorded the steps.
- Check for interference: If you’re pushing a shopping cart, holding a stroller, or keeping your arm unusually still, the watch may miss steps even when you’re walking.
Also check Settings -> Privacy & Security - Location Services ->System Services -> Motion Calibration & Distance is turned on.
More Info : Calibrate your Apple Watch
Measurement: Improve measurement accuracy