The same happened to me in AN11. First when I hit her, I circled back to finish her off and noticed ther was no contact icon on either map

. It was a clear day with calm seas and I could almost touch her from the conning tower. I watched her sink but didn't get a "sunk icon" on the map or a "shes going down" message. No credit either. On a lark I reloaded that patrol, Went back to AN11 and sank a small merchant with the same results. I thought it was the beer at first.