Apple watch not recording steps accurately

The watch makes activity calculations based on what it has that's available. No telling what would make it change, though. You may need to recalibrate it using these steps:

Here's how to reset your calibration data:

1. On your iPhone, open the Apple Watch app.
2. Tap the My Watch tab, then tap Privacy > Reset Fitness Calibration Data. 

See more about that in this article:

Calibrating your Apple Watch for improved Workout and Activity accuracy - Apple Support