Likely because your heart rate is not getting high enough. Check your HR zones.
Not the case .. it dosnt even count with higher heart rate .. but the activity gives credits for restitution.. its not new .. have always been like that . Hr zones are correct
What data you have in this activity statistics?
In my opinion @philipshambrook is right, intensity is to low and your heart rate is to low.
Show us heart rate zones split for this activity.
For me almost all of your activity heart rate would be in Zone1.
Today i set my maximum heart rate on 120 for swim activity .. no count .zones are set for 120 ( just to see if i had any difference)simple Dosnt count for pool swim .. but then again .. todays gym work gave 78 minutes for a 2 hour workout with a average hr on 118 .. but think i will upgrade to a HRM - Pro .. have your guess got activity minutes on a pool swim or are you just guessing
But it gives time for restitution..
I get intensity minutes with to without a strap so perhaps try using the wrist heart rate. I do not have a HRM-Swim. I have HRM-Pro+ but not sure that would make a difference.
Dosnt count for pool swim
The most recent pool swim I did was in June:
Results from my sea swim yesterday...
Lot more data ?? Open Water swim right
I think the main difference is the OWS gets temperature information
I don't know, I have HRM Pro+ strap and all data are there and intensity minutes are counted.
It how it looks on my side: