Speaking from experience with a Fenix 6; only if you set wrist OHR while swimming to On, you get activity minutes for swimming.
You can download the stored heartrate from the Swim HR strap later, but there was an issue at the time (2 year or so ago) that the stored HR was not used for graphs in Connect and not for recalculating training load etc. But those may have been bugs at the time.
Today I swam 2000 meter frontcrawl followed by 500 m breaststroke using F7 with OHR in a 25 meter pool. That generated57 moderate / 29 high = 115 intensity minutes total for the frontcrawl and 1 moderate / 15 high = 31 intensity minutes total for the breaststroke.
I would check the swimming heart rate / zone settings under ‘Sport Heart Rate > Swimming’ in GCM.
If you haven’t already done so, I would try to set the swimming heart rate / zone settings to auto or in such a way that the ‘Time in Heart Rate Zones’ graph reflects your perceived effort as accurate as possible.
I have an Enduro and when I finish swimming it seek my hrm strap and download the data from the strap and then save the activity. Don't know if this makes a difference.
