Welcome aboard
I do know that some people have problems with SH when they save the game whilst out on a mission, usually when there is some 'action' nearby (many players recommend only saving in port or when no other ships are in sight), and it may be this which caused the error, since it would seem to be a problem with the sim's data rather than anything else if there is a sunken ship map icon.
Some players have never had this problem, others have played SH from day one and experienced it, I've only had it once or twice, but the experience of having a screwed up patrol, where my boat would sink five minutes after loading a saved game mid-mission, despite having taken no hits, was enough for me to never save in such circumstances ever again.
My guess is that the data is screwed and you might have to live with it. But
you know you sunk it, so you can be a bit philosophical, not unlike George Peppard in The Blue Max, where they will not confirm his first kill and he is p*ssed off about it, saying; 'But
I saw it, it was a kill!' whereupon he is told: 'Then you have the satisfaction of knowing that you have served your country.'
Unlucky on the other tanker not to get a kill with three fish, but not unknown in reality, with many seperate internal tanks (which can act like watertight compartments) and a lot of reserve buoyancy, if there is no fire, a tanker is a very resilient target.
Chock