Ever since I updated to 4.3, Intensity Minutes are no longer calculated for Workouts.
I contacted Michael @ Garmin support and he suggested that I set my Activity Class in User Settings in the app from 0 to 5 (I have never changed this setting from the default value of 0 until now). I just had another workout, and again, no Intensity Minutes were calculated. Previously to 4.3, Intensity Minutes were being calculated and accumulated during each workout. I am still getting some passive Intensity Minutes for non-workout actions randomly during the day.
My device is a VivoActive 3 with the lasted software/firmware/updates and it had been working fine until Connect 4.3.
I noticed some others in device forums also having the same issue.