Apple Watch not tracking correct workout times

My watch does not record the correct amount of time for certain workouts; I’ve done open water swims for 20 minutes and then come out and it’s showing only 8 workout minutes. Indoor walk also has this problem, if I try to record indoor walk whilst using treadmill, I will walk for 30 minutes and when I get done it will show much less time in the green ring.

If I choose something else like mixed cardio or elliptical machine then it records the correct amount of time. Why is this?