Never has worked that way with any sinkings I've done. Unless the targeted ship receives sufficient damage to trip the "ship destroyed" message you won't get credit for a sinking....and you won't get the ship destroyed message unless you are within a certain range.
Of course this is not peculiar to the NYGM mod but the slow sinking times(some times it is days) of NYMG makes it difficult to hang around in any convoy area, post attack, long enough to get the necessary message.....though with loners you can pull away a bit , submerge to periscope depth,cut your engines, and accelerate time until the ship goes down...even if it is days. You simply can't do that when attacking convoys with escorts.....and I never recall seeing an unescorted one.
Course it may be different if you play with realistic sinking times option
off.... but I wouldn't know about that.
