Someone is stealing my targets

[vedrand]

Hi all,
first of all I want to thank all the people that contributed to GWX. They made this game superb. I used to play SH3 a while ago but after I learned about GWX - I'm amazed and fascinated - I play full real in dead-is-dead mode. I did not return from my first two patrols, but the third career in the med. I like it a lot among other things because of the better weather. In the first patrol I got both periscopes destroyed in an air attack on the second day of the mission...
The current is a little frustrating, but thats life of kaleun, right? For a week I'm fleeing from the bees, I messed up a convoy attack...
Once I got into shooting position of a cargo ship. Aircraft spotted but look... they are ours and they did attack the ship. They damaged it, I though one torpedo should do... it didnt... well
The next day - a lone merchant, nice weather, everything clear for sending it to the bottom (see the screenshot) and rewarding the crew with a bottle of beer each. But then. It just sinks. Just like that, only the gray sinked icon replaces the contact. Now this happened previously as well. What is going on here? It might have been another sub, or friendly planes sinking it. But that is very unlikely in the short span of time. So my actual quesion is - is this a bug or I'm I simply unfortunate...
Thanks all!