Hi
With Wrist Detection and Passcode enabled, check that you are unlocking your watch after placing it on your wrist and that you are wearing your watch sufficiently snugly during your workouts.
Tracking can be affected if your watch loses contact with your skin, causing it to believe that it has been removed from your wrist, leading the Workout app to pause.
A loose fitting band can trigger this by allowing your watch to lift from your wrist and/or by allowing clothing (for example, the end of a sleeve) to slip beneath the back of the watch.
Better contact with your wrist may also improve heart rate sensor performance during your workout.
Apple suggests that you consider tightening the watch band before workouts and loosening it again afterwards.
Sensor performance can also be affected by temporary or permanent changes to your skin, such as some wrist tattoos.
Workouts may also be paused or ended early due to accidental interactions with your watch's screen (including by clothing). To help prevent this, you can lock the screen during workouts (see the Workout app article below for instructions).
It can be normal for the Workout app to pause automatically during indoor and outdoor running workouts when you stop moving and to resume again when you start moving. To turn this feature on or off:
On your iPhone, in the Watch app, go to: My Watch (tab) > Workout - turn on / off Running Auto Pause.
More information:
Use the Workout app on your Apple Watch - Apple Support
Your heart rate. What it means, and where on Apple Watch you’ll find it. - Apple Support