The only time I saw my torp freeze occurred before I had installed any mods other than ship smoke and improved clouds.
I fired at periscope depth and then about twenty seconds later dove. The torpedo halted right about the time my extended scope went under.
Ever get stuck coming up to the bridge in rough weather because the hatch slams on your chest? You're frozen in place until you can manage to click the hatch open again. At this point I'm guessing it is something similar to that - some program glitch triggered by changing depth or crossing the surface plane, because I have not again tried diving until after my torps hit (or miss
) and I have not had it occur again. At least not yet.