If this is the case, it's not a 100% effect. I had missiles that sat there for a good 2-3 minutes while the launching ships acquired other targets (and destroyed them). Missiles in flight seem to change targets 100%, but the stationary ones do not.
I haven't seen this problem in my own game and the situation has occurred a few times. One change in v3.0 is that fire controls are automatically set to no target if they cannot lock their selected target (due to range or target destruction). Not sure how this would cause the missiles to ignore a new target though. Another possibility is if you have multiple fire controls on a ship, make sure the fire control linked to the missiles is the one that selects a new target.
Steve
The designs did include multiple fire controls, but I did retarget both controls to new targets. The only thing I can think of is the incoming missiles took priority, and one or two salvoes was enough to destroy the targets, but flight time was enough to put 4-5 salvoes in flight. So that just compounded the error.
Of course, the other side will start to deploy more PD suites.
Hmm... There's an interesting tactic. Launch oversized salvoes, cease firing and let the existing salvoes take care of business.