Apple Watch (workout) stops tracking exercise mid-walk

Could it be, that the screen has been touched unintentionally or the Digital Crown or Side button pressed during the workout?

To rule that out, enable the Water Lock after the start of the workout via the Control Center, your screen will stay unresponsive until the Digital Crown is pressed again.

In case this is a glitch in the software, try to force restart by pressing and holding the Digital Crown and the Side button together for about 10sec, until the Apple logo comes back again.

If these steps do not work, unpairing the watch would be the next step:

Unpair and erase your Apple Watch - Apple Support

Do you get a reminder, if you want to end the workout, or does the watch just stop recording the exercise?

Autopause is disabled in the settings for Workout in the Watch App on the iPhone?

Wrist Detection is enabled in the settings for passcode, and there are no tattoos beneath the sensors?