Active calories in HIIT work outs greatly reduced

Hello! I’ve been doing HIIT workouts with Apple fitness since it started. I used to get at least 160 (but up to 200) active calories for a 30 min session. In the last couple of months it’s dramatically dropped to 124 active calories for the last sesh I did. I’m pushing myself harder then ever. I’ve noticed I burn more active calories doing 30 mins kick-boxing despite having a lower average heart rate. Has anyone else noticed this? I’ve tried doing my own HIIT workout and it seems to be the same…active calories way down on what I’d expect for a work out of that intensity! I’ve tried resetting calibration but that has made no difference at all. All over workouts give me the active calories I would expect. If anyone can shed some light I’d be grateful!