Depends on what kind of ships they have in those groups, a power of 800 destroyers will easily destroy a power of 800 subs, same if you got 800 subs against 800 cruisers
Tho i'd like you to do a ticket about this from that game so that we can investigate it througth there
I'll mark this post as "On Hold" until i'll be fixed by us from the ticket you'll send in.