I have the first day of week selected as Sunday in the IOS app under More, User Settings but my intensity minutes never show my Sunday workout in the current week. On my watch (Venu SQ) same except when I look at the workout history it does show the Sunday workout in the current week but does not include it in my weekly intensity minutes.
I looked online also at connec.garmin.com but don’t see that as a setting there only in the app.