There is no scenario where the regulator should allow levels as high as 17 Volts. That is likely going to damage any semiconductors, like your LED's, because they are certainly rated for less than 15V max. Even if they don't fail immediately, even a short overvoltage event will shorten their life considerably. It's quite possible series resistances and old wiring connections are reducing voltages in some circuits, so it's not surprising they all don't fail exactly the same time. Either way I wouldn't recommend trying to modify the regulator when they are relatively inexpensive to replace with new. You should probably also replace any LED lights that you think have experienced over-voltage events, or at least check them often and carry spares with you.
Voltage stress is not a linear effect in semiconductors. For example, aging of the light emitting PN junction could accelerate 10X or even much more for each single volt over the design rating, so a bulb run at <15V may last 1000's of hours, 16V may last 100's of hours, 17V may last a day or less, 18V might only last seconds. I make my living stressing iphone semiconductors at elevated voltages to model their reliability and see this sort of thing every day.
All that said, I've had similar sorts of problems with some of these aftermarket regulators failing and allowing both over and under voltage events. My latest solution was to upgrade my alternator to one with internal regulation. I suspect the components used in the internal Bosch regulators are superior and actually designed for reliability.
