Vernersson - can't say conclusively - but I heard Intensity minutes don't get calculated until you have done 10 mins of exercise. So maybe that is the issue
Vernersson You need to have ten minutes of elevated heart rate before Intensity Minutes will start to work.
Here is a useful resource for you to get more information on Intensity Minutes.
https://support.garmin.com/en-GB/?faq=pNU9nnDzzGAHmEavp9rpY8
Vernersson Can we have permission to view your Garmin Connect account so we can look at your settings and activities to get better understand so we can provide you a better answer to your question?
Yes, you can access my GC Account, contact me on my registered e-mail address if necessary.
However, I think that the issue is solved.
Yesterday I updated to beta s/w 9.0.2 and on today's morning run (35 minutes) I actually got intensity minutes from the start. 35min on vigorous intensity = 70 minutes.
The first 10 minutes was also counted.
s/w 8.00 might have a bug?