Sub Stack Can't Attack Again After Oneshot

I have a stack of 10 subs which I cautiously used to approach an enemy battleship and convoy. I was not at war yet, so I waited until my sub stack was right at where the battleship was positioned until declaring war (he had no scout units). After oneshooting the battleships, I went after the convoy stack, which was not far away. However, after entering melee combat with the convoy stack, I realized I had not dealt damage right away, but was on a timer with 15 minutes left until I could deal damage. (It had been 15 minutes since I oneshot the battleship). This means that the attack timer did not reset as it normally does after oneshotting an enemy stack.