I've been having the same issue ever since I got my 3 series watch when it first came out. For several years I tracked my workouts (varied: tennis, running, HIIT, lifting, cycling, swimming) with a Polar H7 chest strap and the motifit app.
My heart rate was always tracked very consistently, and also the readings were instantaneous (i.e. if I was jogging and started sprinting, within 5-6 seconds, I'd see the increase in Heart Rate on my motifit app from, say 155bpm to 170bpm).
Fast forward to using my Apple Watch, and the readings are so inconsistent I stopped even looking at my watch for readings while working out (a main reason why I bought the watch in the first place). I have used several apps to track my heart rate including Zones and Fitiv. With both apps, my heart rate tracking while exercising is wildly inconsistent. For example, I'll be jogging, and my heart rate will read 81bpm, then a minute later 69bpm, when I have done nothing different. I'll start sprinting, and it will take over a minute for the watch to pick up on an elevated heart rate (sometimes it never even picks it up), and even then, sometimes the reading is wildly off- 187bpm (if it ever got that high, I'd probably be in cardiac arrest). I have tried all of the troubleshooting that Apple has recommended: tightening the strap, loosening the strap, wearing it on the other wrist, unpairing/repairing, turning it off/on again. I've tried different straps, cleaned the back of the watch even though it looks clean, and nothing seems to have helped.
It's really frustrating that a watch touted for being a great workout companion can't even monitor your heart rate consistently and accurately.
Yesterday I exercised hard for an hour and 5 minutes, and my the zones app installed on my Apple Watch said I burned 200 calories. Just to compare, the previous day, I did a very similar workout using my old Polar H7 chest strap and it said I burned 450 calories. Come on, Apple...