Missing ship sinking confirmation (resulting in tonnage loss) in captain's logs

[Of the deep end]

Ive only been playing a month, enough to get up to April 1940, but its happened to me once. I had just enough ammo to set a coastal merchant listing but it wouldnt sink so I racked up the time compression. It took about an hour to sink and I didnt get the credit. It must be down to the sinking time.