I am as confused as Garfunkel, but if I read this bit correctly:
attacking one destroyer amongst 70 other ships
it sounds like the issue is not that missiles lose their target but that all of the missiles hit the same target even though only some of them were needed to actually destroy the target?
At least in C#, missiles with sensors do not re-target in the same increment as they would hit the target. If you fire 150 missiles at a target and the "first" 50 hit the target, the next 100 will not re-target and will still hit the same target. As far as I know, this is by design and despite complaints Steve is not looking to change this behavior, as he did not intend for active sensors on missiles to be used to eliminate the ordnance/target management aspect of missile warfare - in other words, Steve wants correctly choosing how much ordnance to dedicate to each target to be a key element of missile-based combat, not something that can be handwaved away with active sensors. You can make "realism" arguments for (and against) this mechanic, but ultimately I think gameplay is the reason it is what it is.
If you fire multiple waves of active sensor-equipped missiles at the same target, and the target is destroyed by the first wave, the second wave will re-target as long as there is indeed a target within range of their active sensors. This, along with targeting at waypoints and two-stage mines (not working; fixed(?) in v2.0), is the intended use of active sensor-equipped missiles.