From: Montreal, Canada
It's been happening to me PBEM++ games with FOW on.
It seems to happen when the diving sub, diving in response to be attacked, moves into, or rather under a hex, containing an enemy unit, whereupon it takes a hit of one damage as though it were a ship moving through a hex containing another hidden ship, but then re-emerges after diving, with the hit point restored. It's not a huge problem more than it is a anomaly of the game mechanics, since the sub doesn't really take any damage in the end.
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