Issue with swimming tracking and auto-rest

So I had an Apple Watch and moved to a Descent MK3i because I’m a freediving instructor and this watch covers all my needs in one device.

However, with the AppleWatch, i put it on, set the pool length and off i go, it automatically measures and creates sets, can detect rest etc and was very accurate.

With the Garmin now, I had to enable auto-rest so that it can detect my stops etc but it seems to be missing lengths quite a lot.

Today for instance I did 1050m but the watch measured 975. Oddly enough it failed to detect 25m from a 200m freestyle set, then another 25m from a 100m set and at the end a 25m breaststroke.

This seems borderline unacceptable for a €1600 smart watch, is there something I’m doing wrong in the settings maybe?

Thanks for any help!