My impression is bugs related to sunk vessels not really recognized as sunk have been an on-going problem. The latest patch beta addresses another one similar to this. In your case, there is the possibility the first torp was not a totally lethal hit (similar to what might happen to yourself if a hit caused enough flooding to bring you to the bottom) and your crew was still alive and repairing, trying to save the boat although it likely would never go offensive again. I don't know if the simulation supports near misses where the boat floods but doesn't get lethal damage.
This might be why your crew detected the sub as still "active" but not moving whereby another torp was required to seal the fate of the boat. What I didn't see in your report is if you had a sunk icon on the map for that contact which might be the deciding factor. Keep hitting it until you get the sunk icon.
As stated in a famous movie ... Ships made to sink are hard to sink.
-Pv-
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