I think I already read about this issue some times ago, but it seems to still be here and I can’t seem to find the old thread.
So basically, stacking different shields on a ship boosts the shield resistance of every one of them up to the value of the strongest one. So, if you mix light shield generators and reflective shields, the light shields, when hit, will have a resistance of 27.
Thats a feature not a bug. But as I recall, this was fixed insomuch as the resistance of a ship at any given point is the highest resistance amongst its working shields. So if you have 2 generators and 1 is destroyed, your resistance may fall, if they ahve different values.