[UnderseaLcpl]

As far as I know, it is quite possible to hit a ship, set it alight, and then watch it steam away as though little or nothing had happened, even if it develops a list. The convoy could have altered speed as well. I know for a fact that they make frequent course/speed changes in GWX, sometimes even during an attack. That would account for other ships passing the tanker.
I don't think that is a bug.

As far as the lost renown goes, ships that are damaged can be sunk by high seas. As far as the game is concerned, you didn't sink it, the ocean did. What was the weather like?
It could also be that another U-boat or an aircraft showed up and sank it, though that seems unlikely.