Not sure myself mate.Good to know someone else has had this happen.Someone in here will no doubt know what is going on here,probably somebody back at base not doing their job properly is my guess
I was definitely close enough to the last sinking,approx 1500-2000m off its beam after a night surfaced attack if I remember correctly.I also remember it taking a while to go under,approx 30 mins and I stayed around until it went under then radioed base that I was returning after all torpedoes were expended.
As I said,the in game log (button k) confirmed all 3 kills.